Class 9 Science

Matter In Our Surroundings

In our surroundings, we see a large variety of objects with different shapes, sizes and textures. All objects everything in this universe is made up of material which scientists have named ‘matter’.

• The air we breathe, the food we eat, stones, clouds, stars, plants and animals, even a small drop of water or a particle of sand— everything is matter. Matter can be seen, tasted, smelled or felt.
• Matter can neither be created nor destroyed, it can only be changed from one form to another. In the modern day, scientists have evolved two types of classification of matter based on their physical and chemical properties. In this chapter, we shall learn about matter based on its physical properties.

Matter

Matter is anything that has mass and volume or we can say that anything that has mass, occupies space and can be felt by one or more sense organs is called matter.

Note: The SI unit of mass is kilogram (kg), volume is cubic metre (m³). The common unit of measuring volume is the litre (L) and 1L = 1 dm³, 1L = 1000 mL, 1 mL =1 cm³

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Classification Of Matter

1. Early Indian philosophers classified matter into five basic elements, called the Panch-Tatva. These are air, water, earth, sky and fire. According to them everything living or non-living, was made up of these five basic elements.
2. Nowadays, matter is classified according to its physical properties and chemical nature.

For example., solid, liquid and gas (based on particle arrangement or physical properties) or elements, compounds and mixtures (based on chemical nature).

Physical Nature Of Matter

If we study the physical composition of matter, we find that:

1. Every matter is made up of certain particles which differ in shape, size and nature from other types of matter.
2. The particles of matter are tiny (beyond our imagination).

Characteristics Of Particles Of Matter

Some important characteristics of particles of matter are as follows:

1. Particles of matter have space between them.
2. Particles of matter are in a state of continuous movement. This suggests that they possess some energy, called kinetic energy. As the temperature rises, the kinetic energy of the particles increases and hence, particles move faster.
3. The particles of matter tend to diffuse, i.e. to intermix on their own with each other. They do so by getting into the spaces between the particles. The intermixing of particles of two different types of matter on their own is called diffusion.
4. Particles of matter attract each other. A force of attraction exists between the particles, which is known as the intermolecular force of attraction. This force keeps the particles together. The strength of this force of attraction varies from one kind of matter to another.

Diffusion And Osmosis

Diffusion is the process in which molecules of a substance move from higher concentration to lower concentration and goes on until a uniform mixture is formed. In osmosis, the solvent molecules move from their lower concentration to higher concentration through a semipermeable membrane.

States Of Matter

Matter around us exists in three different states which are solid, liquid and gas. These states of matter arise due to the variation in the characteristics of the particles of matter.

The Solid State: A Solid is defined as that form of matter which possesses rigidity, incompressible and hence, has a definite shape and a definite volume.

Some Important Properties Of Solid State Are As Follows:

1. Solids have definite shapes, distinct boundaries and fixed volumes, i.e. have negligible compressibility.
2. Solids tend to maintain their shape when subjected to outside force.
   1. Exception A rubber band changes shape under force and regains the same shape when the force is removed. If excessive force is applied, it breaks.
   2. Sugar and salt also take the shape of the container in which they are placed but are considered solids.
   3. This is because the shape of each sugar or salt crystal remains fixed.
3. Solids either do not diffuse or diffuse at a very slow rate.
   1. Exception Sponge is compressible but considered as a solid. This is because a sponge has minute holes, in which air is trapped.
   2. When it is pressed, the air is expelled and we can compress it.
4. Solids may break under force, but it is difficult to change their shape, so they are rigid.
5. Generally, solids have higher densities as compared to their liquid or gaseous forms.
   1. Sugar, sand, rocks, stones, and metals like iron, copper, aluminium, gold, silver, etc., are examples of substances which exist in the solid state.

The mass per unit volume of a substance is called its density,

∴ Density =\(\frac{\text { Mass }}{\text { Volume }}=\frac{m}{V}\)

The Liquid State: Liquid is defined as that form of matter, which possesses a fixed volume, but has no fixed shape.

Some Important Properties Of the Liquid State Are As Follows:

1. Liquids do not have a definite shape, i.e. they take up the shape of the container in which they are kept.
2. Liquids flow and change shape, so they are not rigid but can be called fluid.
   1. Note: Fluid In science, the common name of gases and liquids is fluid.
3. Solids, liquids and gases can diffuse into liquids. The gases from the atmosphere diffuse and dissolve in water. These gases, especially oxygen and carbon dioxide, are essential for the survival of aquatic animals and plants. Aquatic animals can breathe underwater due to the presence of dissolved oxygen in water.
4. Liquids are almost incompressible.
5. The attraction force between the particles of liquid is greater than that of gases but less than that of solids.
6. The rate of diffusion of liquids is higher than that of solids. This is because, in the liquid state, particles move freely and have greater space between each other as compared to particles in the solid state.
7. The density of a liquid is generally less than that of its solid form. Some exceptions are also there, for example., solid ice is lighter than water as it floats on water, i.e. the density of the solid form of water (ice) is less as compared to that of the liquid form of water. Water, milk, juice, oil, kerosene, petrol, alcohol, benzene etc., are examples of the substances which exist in the liquid state.

The Gaseous State: Gases can be defined as that form of matter which possesses high compressibility and hence, has neither definite shape nor definite volume.

Some Important Properties Of Gaseous State Are As Follows:

1. Gases tend to flow as liquids do. Therefore, they are also considered as fluids.
2. Gases show the property of diffusing very fast into other gases due to the high speed of particles and the large spaces between them.
   1. Due to the high diffusion tendency of gases, the smell of hot cooked food reaches us in seconds. The particles of the aroma of food mix with the particles of air spread, reaching us and even farther away.
3. Gases are highly compressible. The Liquefied Petroleum Gas (LPG) cylinder used in our homes for cooking or the oxygen supplied to hospitals in cylinders is compressed gas.
   1. Compressed Natural Gas (CNG) is used as a fuel these days in vehicles. Due to its higher compressibility, large volumes of gas can be compressed into a small cylinder and transported easily.
4. In a gaseous state, the particles move about randomly at high speed. Due to this random movement, gases exert pressure on the walls of the container, in which they are kept. Air is an example of a gaseous state. It is a mixture of gases like oxygen, nitrogen, carbon dioxide, inert gases, etc. Other examples of gases are hydrogen, ammonia, nitrogen dioxide, sulphur dioxide, etc.
5. All living creatures need to breathe for survival. So, solids, liquids and gases can diffuse into liquids.
6. The density of gases is minimal. A gas is much lighter than the same volume as a solid or a liquid.

Rigidity And Fluidity

Rigid means inflexible. A solid is a rigid form of matter, hence it does not require a container to keep it. Fluid is a material which can flow easily and requires a vessel to keep it. A liquid is a fluid form of matter which takes the shape of a container, while a gas is a fluid form of matter which fills the container.

Change Of States Of Matter

In your daily life, you come across various substances which exist in three states, i.e. solid, liquid and gas, for example. water, wax, ghee, etc. Water is the most commonly observed example that exists as ice (solid), water (liquid) as well as water vapour (gas).

Interconversion Of States Of Matter: The states of matter are interconvertible. The phenomenon of change of matter from one state to another and back to the original state by altering the conditions of temperature and pressure is called the interconversion of states of matter.

The following two factors (or any one of these) make it possible to convert one state of matter into another:

1. Change in temperature
2. Change in pressure

Terms Involved In Change Of State

The following terms are involved in a change of state:

1. Fusion or Melting and Melting Point
   1. The process of conversion of a matter from its solid state to its liquid state at specific conditions of temperature and pressure is called fusion/melting.
   2. The definite temperature at which a solid starts melting is called the melting point of that solid, for example., the melting point of ice is 0°C or 273.16 K. The Higher the melting point of a substance, the greater the force of attraction between its particles.
2. Boiling and Boiling Point
   1. The process of conversion of a matter from its liquid state to vapours (gaseous state) at specific conditions of temperature and pressure is called boiling.
   2. It is a bulk phenomenon. The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point.
3. Sublimation
   1. The process of changing of solid state directly into a gaseous state without passing through the liquid state upon heating and vice-versa on cooling is known as sublimation. for example., naphthalene, camphor, iodine, ammonium chloride, etc., are the solids that undergo sublimation.
4. Vapourisation
   1. The process of conversion of a matter from its liquid state to a gaseous state at specific conditions of temperature and pressure is called vapourisation.
5. Freezing and Freezing Point
   1. The process of conversion of matter from its liquid state to solid state at specific conditions of temperature and pressure is called freezing.
   2. It is a reverse process of fusion/melting. The definite temperature at which a liquid changes into a solid state by giving out heat energy at 1 atm is called the freezing point.
6. Condensation
   1. The process of conversion of matter from its gaseous state to liquid state at specific conditions of temperature and pressure is called condensation. It is a reverse process of vapourisation.

Effect Of Change Of Temperature

When a solid is heated, the kinetic energy of its particles increases. Due to an increase in kinetic energy, the particles start vibrating with greater speed.

• The energy supplied by the heat overcomes the forces of attraction between the particles.
• The particles leave their positions and start moving more freely. At a certain stage (i.e. at a melting point), a solid melts and is converted into a liquid state.
• At a certain temperature, a point is reached when the particles have enough energy to break free from the forces of attraction of each other. At this temperature (i.e. boiling point), the liquid starts changing into gas.
• In contrast, by decreasing the temperature (by cooling), a gas can be converted into a liquid state and a liquid can be converted into a solid state.

The effect of change in temperature on the physical state may be summarised as:

So, it can be concluded that the state of matter can be changed into another by changing the temperature.

Difference Between Gas And Vapour

A substance is said to be a gas if its boiling point is below room temperature, for example., O₂, N₂, CO₂, etc. If the normal physical state of a substance is either a solid or a liquid, but gets converted into the gaseous state either on its own or by absorbing energy, the gaseous state is called the vapour state, for example., vapours of water in air.

Scales of Measuring Temperature

Three scales of measuring temperature are as follows:

1. Temperature on Kelvin scale

= Temperature on Celsius scale +273.16;

T(K)=t(°Q +273.16

Kelvin is the SI unit of temperature, 0 °C =273.16 K

For convenience, we take 0°C =273 K

2. Temperature on Celsius scale

= Temperature on Kelvin scale -273.16;

t(°C=T(K)-273.16

3. Temperature on Fahrenheit scale: Celsius and Fahrenheit temperatures are related to each other by the relation,

∴ \({ }^{\circ} \mathrm{F}=\frac{9}{5}\left({ }^{\circ} \mathrm{C}\right)+32\)

Question 1. Convert the temperature of 200°C to the Kelvin scale.
Answer:

We know that, the temperature on the Kelvin scale

Temperature on Celsius scale +273.16 = 200 + 273.16 = 473.16 K

Thus, a temperature of 200°C on the Celsius scale is equal to 473.16 K on the Kelvin scale.

Question 2. Convert the temperature of 450 K to the Celsius scale.
Answer:

We know that, the temperature on the Celsius scale

= Temperature on Kelvin scale – 273.16

= 450-273.16 =176.84°C

Thus, a temperature of 450 K on the Kelvin scale is equal to 176.84°C on the Celsius scale.

Latent Heat

When heat is given to a substance, its temperature increases. However, when heat is supplied to change the physical state of a substance, there is no increase in the temperature of a substance.

• Thus, the heat energy which has to be supplied to change the state of a substance is called its latent heat. In actuality, the word latent’ means ‘hidden’.
• Latent heat does not raise (or increase) the temperature. However latent heat is always supplied to change the state of a substance.

Latent Heat Is Of The Following Two Types:

Latent Heat Of Fusion (Solid To Liquid Change)

The amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure and at its melting point is known as the latent heat of fusion. Particles in water at 0°C (273.16 K) have more energy as compared to particles in ice at the same temperature.

Latent Heat Of Vapourisation (Liquid To Gas Change)

The amount of heat energy that is required to convert 1 kg of a liquid into gas (at its boiling point) without any temperature rise is known as the latent heat of vapourisation. Particles in steam, i.e. water vapour at 373 K (100°C) have more energy than water at the same temperature.

Note: It has been found that burns caused by steam are much more severe than those caused by boiling water though both of them are at the same temperature of 100°C.

As particles in steam have absorbed extra energy in the form of latent heat of vapourisation. Thus, when steam falls on our skin and condenses to produce water, it gives more heat than boiling water.

Effect Of Change Of Pressure

The physical state of a substance can also be changed by changing the pressure. An increase in pressure brings the particles closer and increases the force of attraction between them, which brings about the change, for example., when high pressure is applied to a gas and its temperature is reduced, the gas is converted to a liquid, i.e. the gas is liquefied.

Hence, we can say that pressure and temperature determine the state of a substance, whether it will be solid, liquid or gas.

The pressure exerted by a gas is measured in the atmosphere (atm) unit. The pressure of air in the atmosphere is called atmospheric pressure.

Atmospheric pressure at sea level is taken as 1 atm which is also normal atmospheric pressure. As we go higher, atmospheric pressure decreases.

1 atm = 1.01 x 10⁵ Pa (Pa = Pascal, SI unit of pressure)

Evaporation

The process of conversion of a liquid into its vapour state at any temperature below its boiling point is called evaporation. The particles of a liquid have different amounts of kinetic energy.

• The particles present at the surface possess comparatively higher kinetic energy as compared to those present in the bulk.
• Therefore, particles at the surface with higher kinetic energy can break away from the forces of attraction of other particles and get converted into vapour.
• Water, when left uncovered, slowly changes into vapour. Wet clothes dry up, etc., are happen due to evaporation.

Factors Affecting Evaporation

The rate of evaporation of a liquid depends upon the following factors:

1. Surface area Evaporation is a surface phenomenon, if the surface area is increased, the rate of evaporation increases, for example., while putting clothes for drying up, we spread them out.
2. Temperature The rate of evaporation of a liquid increases with a temperature rise. With the increase of temperature, more particles get enough kinetic energy to go into a vapour state. That is why, evaporation is faster in a hot summer day than in winter or on a cloudy day.
3. Humidity It is the amount of water vapour present in air. The air around us cannot hold more than a definite amount of water vapour at a given temperature. If the amount of water in air is already high, the rate of evaporation decreases. That is why, clothes dry up faster on a dry day than on a wet (rainy) day.
4. Wind speed It is known that clothes dry faster on a windy day. This is because, with an increase in wind speed, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surroundings. That is why, the rate of evaporation of a liquid increases with increasing wind speed.

Note: The liquids which evaporate fast are called volatile liquids.

Evaporation Causes Cooling Effect

In an open vessel, the liquid keeps on evaporating. The particles of liquid absorb energy from the surroundings to regain the energy lost during evaporation. This absorption of energy from the surroundings makes the surroundings cold.

Some daily life examples of the cooling effect of evaporation are given below:

1. When ice-cold water is kept in a glass tumbler for some time, water droplets are observed on its outer surface.
   1. Explanation This occurs as the water vapours present in the air come in contact with the glass tumbler, get cooled and condense to form these small water droplets.
   2. The formation of drops of water on the outside surface of a tumbler containing crushed ice shows the presence of water vapour in the air.
2. Cotton clothes are used to wear during the summer season.
   1. Explanation Cotton is a good absorber of water, so it helps to absorb sweat from our bodies.
   2. As it is obvious, the person perspires more during summer due to the auto temperature control mechanism.
   3. Hence, wearing cotton clothes helps in the easy evaporation of sweat.
   4. When this sweat evaporates, it takes the latent heat of vapourisation from our body, which in turn, cools the body. Thus, a person feels comfortable.
3. People sprinkle water on the roof or open ground on a hot sunny day.
   1. Explanation When water is sprinkled on a hot surface, it gets evaporates very quickly.
   2. As evaporated water leaves the surface cool due to the large latent heat of vapourisation of water, this technique is quite effective in summers for cooling the surface.
4. Liquids like acetone (nail polish remover) or alcohol placed on your palm give you a feeling of cooling.
   1. Explanation Acetone and alcohol are volatile liquids. When kept on the palm, their particles gain energy from the palm or surroundings and evaporate causing the palm to feel cool.

Activity 1

Objective

To prove that matter is made up of tiny particles (and have intermodular space).

Materials Required

Beaker, water, salt or sugar, glass rod and marker.

Procedure

1. Take a 100 mL beaker half filled with water and mark the initial water level with the help of a marker.
2. Then, add a teaspoonful of sugar (or salt) into it and stir with the help of the glass rod.
3. Mark the water level after the disappearance of the solute.

Question 1. What happens to the sugar when it is dissolved in water?
Answer: When sugar is dissolved in water, its crystals separate into very fine particles.

Question 2. Where does the sugar go?
Answer:

The sugar particles go into the spaces present between the particles of water and mix with them to form a sugar solution.

Activity 2

Objective

To prove that particles of matter:

1. Are very small in size (particulate in nature).
2. Move (diffuse) faster in a gaseous state as compared to solid or liquid states.
3. Diffuse at a slower rate, if the density is higher.
4. Diffuse faster at a higher temperature

Materials Required

Potassium permanganate, water, beakers, perfume, blue ink, honey and copper sulphate.

Observation

The water level does not change.

Explanation

• Matter is not continuous and is particulate, i.e. it is made up of particles. When salt is dissolved in water the water level does not change.
• It indicates that there are some vacant spaces among the particles of water. These are known as interparticle spaces. The particles of salt have occupied some of them.

Conclusion

Matter is made up of tiny particles and intermolecular spaces are present in between them. Check Yourself

Observation Table

Question 1. What conclusion can you draw after adding 2-3 crystals of KMnO₄ in water?
Answer:

After the addition of 2-3 crystals of KMn04 in water, it is concluded that a crystal of KMnO₄ is made up of millions of tiny particles. They keep dividing themselves into smaller particles.

Question 2. When someone opens a bottle of perfume in one corner of a room, its smell spreads in the whole room quickly. Why?
Answer:

This happens because the particles of perfume (gas) move rapidly in all directions and mix with the moving particles of air in the room. They do so by getting into the spaces between the air particles.

Question 3. Why honey in step 3 dissolves at a slower rate?
Answer:

In step 3, honey dissolves at a slower rate because it is more viscous i.e. has more density and has strong intermolecular forces of attraction.

Question 4. From step 4, write the effect of temperature on diffusion.
Answer: Diffusion becomes faster at a higher temperature.

Question 5. The rate of diffusion is faster in gases, why?
Answer:

The molecules of gases have large intermolecular space between them and have higher speeds. So they diffuse faster.

Activity 3

Objective

To study the properties of solids and liquids.

Materials Required

Pencils, books, needles, thread, paper, hammers, some liquids (for example., water, oil, milk etc), containers of different shapes and the same volume.

Procedure

1. Take a book, a needle and a piece of thread. Draw their sketch on paper with the help of a pencil. Observe their shapes and judge their volume. Now pull, drop and hammer these things one by one and record your observations.
2. Take some liquids (50 mL each) and pour them into containers of different shapes. Put a 50 mL mark on these containers using a measuring cylinder from the laboratory.

Observe the shapes of each liquid in each container. When 50 mL of liquid is poured into another container, does its volume change?

Observation

1. All these things (books, needles and thread) have sharp boundaries, i.e. definite shapes and definite volumes. When pulled, dropped or hammered, these things remain unaffected or do not break.
2. Volume of the liquids remains the same but its shape depends upon the shape of the container.

Conclusion

• Solids have a definite shape and definite volume. Solids are hard and rigid and held together with greater force.
• Liquids have fixed volumes but different shapes, as they acquire the shape of the container in which they are kept Liquids tend to flow, i.e. these are fluids.

Question 1. In which states of matter, the arrangement of particles is in the most ordered form?
Answer: In a solid state, the particles are arranged in the most ordered form.

Question 2. State one similarity between solids and liquids.
Answer: Both solids and liquids have a definite volume.

Question 3. What is the nature of shape and volume of liquids?
Answer: Liquids have indefinite shape and definite volume.

Activity 4

Objective

To show that gases can be compressed more easily than liquids and solids.

Materials Required

Three syringes, rubber corks, chalk and Vaseline.

Procedure

1. Take three 100 mL syringes and close their nozzles by rubber corks, as shown in the figure below.

Compressing gas, liquid and solid on applying pressure

2. Remove the pistons from all the syringes.

3. Leaving the first syringe untouched, fill water in the second and pieces of chalk in the third.

4. Insert the pistons back into the syringes. You may apply some Vaseline on the pistons before inserting them into the syringes for their smooth movement.

5. Now, try to compress the content by pushing the piston in each syringe.

Observation

The piston of the first syringe can be moved very easily but less easily in the second and with the most difficulty in the third.

Conclusion

Gases are highly compressible as compared to solids and liquids.

Question 1. Why gases can be compressed more easily than liquids?
Answer:

Cases can be compressed more easily than liquids because of weak intermolecular forces of attraction between particles.

∴ \(\left(\text { Because, Compressibility } \propto \frac{1}{\text { Intermolecular forces of attraction }}\right)\)

Question 2. In this activity, what do you infer from your observations?
Answer: Gases are highly compressible as compared to solids and liquids.

Question 3. Why solids cannot be compressed?
Answer:

Solids cannot be compressed because constituent particles are very closely packed and the movement of constituent particles is restricted.

Question 4. Which form of water is highly compressible?
Answer: The gaseous form of water is highly compressible.

Activity 5

Objective

To show the effect of temperature on the physical state of matter.

Materials Required

Ice, beaker, laboratory thermometer, glass stirrer, burner and iron stand.

Procedure

1. Take about 150 g of ice in a beaker and suspend a laboratory thermometer in it such that the bulb of the thermometer is in contact with the ice.
2. Start heating the beaker on a low flame.
3. Note the temperature when the ice starts melting.
4. Note the temperature when all the ice has converted into water.
5. The observations are recorded for the conversion of ice into water.
6. Place a glass stirrer in the beaker and heat while stirring till the water starts boiling.
7. Note the temperature when most of the water vaporised.

Observation

1. There is no change in temperature till all the ice melts though heating continues.
2. Temperature remains constant at 0°C. Once the ice is converted to water, the temperature starts rising till the water begins to boil. Once the water starts boiling, the temperature remains constant at 100°C till all the water has changed to vapours.

Conclusion

• During the change of state from solid to liquid or from liquid to gas, the temperature remains constant till all the solid has melted or all the liquid has vaporised.
• The heat energy supplied is used up in overcoming the forces of attraction and hence, the thermometer does not show any temperature rise.

Question 1. What is the change in temperature during this activity?
Answer: The temperature remains constant during the complete melting and boiling process.

Question 2. Is melting an exothermic or endothermic process?
Answer: Melting is an endothermic process because heat is absorbed during this process.

Question 3. Under which conditions, we can boil water at room temperature?
Answer: We can boil water at room temperature under low pressure.

Question 4. Draw a temperature-time graph for the heating of ice.
Answer: The graph of temperature-time for the heating of ice is given below:

Question 5. In an experiment for studying the effect of heating on ice, why do we use crushed ice?
Answer: Crushed ice will cover the thermometer bulb intimately and thus, would give the correct temperature.

Question 6. What do we call the temperature at which liquid starts boiling?
Answer: The temperature at which liquid starts boiling is called its boiling point.

Question 7. What would be the temperature when water starts boiling?
Answer: As the water starts boiling, the temperature will be 100°C or 373K.

Question 8. What is the temperature when all the water has boiled?
Answer: The temperature remains constant, i.e. 100°C during the complete boiling process.

Activity 6

Objective

To study the process of sublimation.

Materials Required

Camphor or ammonium chloride, China dish, funnel, cotton plug and burner.

Procedure

1. Take some camphor or ammonium chloride. Crush it and put it in a China dish.
2. Put an inverted funnel over the China dish.
3. Put a cotton plug on the stem of the funnel and set the apparatus

Observation

Solid ammonium chloride changes into vapours without changing into a liquid state and gets condensed on the walls of the funnel.

Conclusion

A change of state directly from solid to gas without changing into liquid state or vice-versa is called sublimation.

Question 1. In an experimental set-up for sublimation, a perforated asbestos sheet is placed between the China dish and funnel. What is its purpose?
Answer: The asbestos sheet prevents direct heating of the funnel.

Question 2. What happens when solid ammonium chloride is heated?
Answer:

On heating, solid ammonium chloride changes into vapours without changing into a liquid state and gets condensed on the walls of the funnel,

Question 3. Name the solid substance which is obtained by cooling the vapour.
Answer: The solid substance obtained by cooling the vapour is known as sublimate.

Question 4. Which substance is sublimated in this activity?
Answer: Sublimate is pure ammonium chloride.

Activity 7

Objective

To study the factors which affect evaporation.

Materials Required

Test tubes, water, jar, China dish, thermometer and cupboard.

Procedure

1. Take 5 mL of water in a test tube and keep it near a window or under a fan.
2. Take 5 mL of water in an open China dish and keep it near a window or under a fan.
3. Take 5 mL, of water in an open China dish and keep it inside a cupboard or on a shelf in your class.
4. Record the room temperature.
5. Record the time or days taken for the evaporation process in the above cases.
6. Repeat the above three steps of activity on a rainy day and record your observations.

Observation

1. In an open China dish kept near a window or on a shelf, water evaporates very fast as compared to a test tube.
2. On sunny days, water evaporates very fast as compared to rainy days.

Conclusion

• Surface area is increased the rate of evaporation increases.
• If the temperature is increased, the rate of evaporation increases because with the increase of temperature, more particles get enough kinetic energy to go into the vapour state.
• If wind speed is increased, the rate of evaporation increases because with the increase in wind speed, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surroundings.
• If humidity is decreased, the rate of evaporation increases and vice-versa

Question 1. Why does water evaporate faster in a Chinese dish as compared to a test tube?
Answer:

China dish has more surface area as compared to a test tube. So, evaporation is faster in the case of China dishes.

Question 2. Why evaporation is fast when water is kept under a fan?
Answer:

When water is kept under a fan, the rate of evaporation increases because with the increase in wind speed, the particles of water vapour move away with the wind.

Question 3. What is the effect of humidity on evaporation?
Answer: If humidity is decreased, the rate of evaporation increases and vice-versa.

Question 4. What do you infer about the effect of temperature on evaporation?
Answer: If the temperature is increased, the rate of evaporation increases.

Question 5. Why evaporation is slow on a rainy day?
Answer: On a rainy day, humidity is increased, so the rate of evaporation decreases.

Matter In Our Surroundings Summary

Matter is anything that has mass and occupies volume. The SI unit of mass and volume is kilogram (kg) and cubic metre (m3), respectively.

• Matter is classified based on their physical and chemical properties, i.e. physical properties (solid, liquid and gas) and chemical properties (elements, compounds and mixtures).
• Every matter is made up of certain particles which differ in shape, size and nature.
• The particles of matter tend to diffuse.
• Solids have definite shapes, distinct boundaries and fixed volumes.
• Liquids do not have a definite shape.
• Gases have neither definite shape nor volume.
• The state of matter can be interchanged by changing temperature or pressure.
• At specific conditions of temperature and pressure; the conversion of a matter from its solid to its liquid state is called fusion.
• The conversion of a matter from its liquid state to vapour (gaseous state) is called boiling.
• It is a bulk phenomenon.
• The conversion of a matter from its liquid to solid state is called freezing.
• The conversion of a matter from its liquid to gaseous state is called vapourisation.
• The conversion of matter from its gaseous to liquid state is called condensation.
• The process of changing of solid state directly into a gaseous state without passing through the liquid state upon heating and vice-versa on cooling is called sublimation. The heat energy which has to be supplied to change the state of a substance is called latent heat.
• The latent heat of vapourisation is the heat energy required to change 1kg of a liquid to gas at atmospheric pressure at its boiling point.
• Latent heat of fusion is the amount of heat energy required to change 1 kg of solid into liquid at its melting point.
• The process of conversion of a liquid into its vapour state at any temperature below its boiling point is called evaporation.
• Apart from the solid, liquid and gaseous states, scientists have discovered two more states, i.e. plasma and Bose-Einstein condensate.

Is Matter Around Us Pure

In chemistry, when we say a substance is pure, it means that the substance is made up of only one type of constituent particle. In other words, a substance is a pure single form of matter.

Depending upon the chemical composition, matter is classified into elements, compounds, (i.e. pure substances that are non-separable by physical methods) and mixtures (separable by physical methods like sublimation, etc).

Pure Substance

A substance that consists of only a single type of constituent particles is called pure substance, for example., gold, water, etc. Based on the nature of the constituent particles, a pure substance is of two types, i.e. elements and compounds.

1. Elements

The term element was first used by Robert Boyle in 1661. According to Antoine Laurent Lavoisier, a French chemist, (1743-94), ‘an element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions’.

Read and Learn  More Class 9 Science Notes

An element is a pure substance. Till now 118 elements have been discovered, out of these 92 are natural elements and others are man-made. Based on variation in properties, elements can be broadly classified as metals, non-metals and metalloids.

1. Metals
   1. A metal is an element that is malleable (i.e. can be hammered into thin sheets), ductile (i.e. can be drawn into wires), sonorous (i.e. make a ringing sound when hit), and conduct heat and electricity.
   2. They are lustrous (shine) and have silvery-grey or golden- yellow colour, for example., gold, silver, copper, iron, sodium, potassium, etc.
   3. Mercury is the only metal that is liquid at room temperature. Gallium and caesium because of their very low melting points remain in liquid state at a temperature slighdy above room temperature (303K).
2. Non-metals
   1. A non-metal is an element that is neither malleable nor ductile and does not conduct heat and electricity.
   2. They display a variety of colours, for example., hydrogen, oxygen, iodine, carbon (coal, coke), bromine, chlorine, etc.
   3. Bromine is the only non-metallic element that exists in a liquid state at normal conditions of temperature and pressure.
3. Metalloids
   1. Elements having intermediate properties between those of metals and non-metals are called metalloids, for example., boron, silicon, germanium, etc.

2. Compounds

A compound is a substance composed of two or more elements, chemically combined in a fixed proportion, for example., water (H₂O), methane (CH₄), carbon dioxide (CO₂), ammonia (NH₃), sodium chloride (NaCl), etc.

Mixtures

Mixtures are constituted by more than one kind of pure form, known as a substance. Most of the matter around us exist as mixtures of two or more pure components, for example., sea water, minerals, soil, etc, are all mixtures.

Types of Mixtures: Depending upon the nature of the components that form a mixture, we have two types of mixtures:

1. Homogeneous Mixture
   1. A mixture in which the constituents are uniformly distributed throughout i.e. without any clear boundary of separation is called homogeneous mixture.
   2. Here, the constituents cannot be seen with naked eyes or under a microscope.
   3. Some of the examples of homogeneous mixtures are salt solution, sugar solution, air, soft drinks, petroleum, biogas, alloys, etc.
   4. Note Air is a homogeneous mixture of gas. Its two major constituents are oxygen (21 %) and nitrogen (78%) and other gases in small quantities.
2. Heterogeneous Mixture
   1. A mixture that does not have uniform composition, i.e. has visible boundaries of separation between its constituents is called heterogeneous mixture.
   2. Here, the constituents of a heterogeneous mixture can be seen by naked eyes or under a microscope.
   3. Examples of heterogeneous mixtures are sugar and sand mixture, salt and sand mixture, polluted air, muddy water, etc.

Differences Between Compounds And Mixtures:

Solution

A homogeneous mixture of two or more substances is called a solution. A solution is sometimes also called a true solution.

• Lemonade, soda water, salt solution, sugar solution, etc., all are examples of solutions.
• In a solution, there is homogeneity at the particle level, i.e. the particles of dissolved substances are evenly distributed in the solution and are indistinguishable from one another.

There are two main components of a solution:

1. Solvent (Dissolving Phase) The component (usually present in larger amounts) of the solution that dissolves the other component in it, is called the solvent.
2. Solute (Dissolved Phase) The component (usually present in lesser quantity) of the solution that is dissolved in the solvent is called the solute.

Some Common Examples Of Solution:

1. In sugar solution, sugar is the solute and water is the solvent.
2. A solution of iodine in alcohol known as tincture of iodine, has iodine (solid) as the solute and alcohol (liquid) as the solvent.
3. Aerated drinks like soda water, etc., are gas in liquid solutions. CO₂ (gas) as solute and water (liquid) as solvent. Solid solutions (alloys) and gaseous solutions (air)

Alloys

• Alloys are mixtures of two or more metals or a metal and a non-metal and cannot be separated into their components by physical methods.
• But still, an alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition, for example., brass is a mixture of approximately 30% zinc and 70% copper.

Properties Of A Solution

Some important properties of a solution are as follows:

1. A solution is a homogeneous mixture.
2. The particles of a solution are smaller than 1 nm (10-9 m) in diameter. Therefore, they cannot be seen by the naked eye.
3. Due to very small particles, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution.
4. A solution is stable, i.e. the solute particles do not settle down when left undisturbed. The solute particles cannot be separated from the mixture by the process of filtration.

Concentration Of A Solution

• The concentration of a solution is the amount of solute present in a given amount (mass or volume) of solution, or the amount of solute dissolved in a given mass or volume of the solvent. In a solution, the relative proportion of the solute and solvent can be varied.
• Depending upon the amount of solute present in a given amount of solvent, it can be classified as under:

Saturated solution A solution in which no more amount of solvent can be dissolved at a given temperature, is called a saturated solution. The amount of the solute present in the saturated solution at this temperature is called solubility.

Solubility =\(\frac{\text { Mass of solute }}{\text { Mass of solvent }} \times 100\)

Unsaturated solution If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution.

Concentration of solution

⇒ \(=\frac{\text { Amount of solute }}{\text { Amount of solution }}=\frac{\text { Amount of solute }}{\text { Amount of solvent }}\)

Expressing The Concentration Of A Solution: The methods by which the concentration of a solution can be expressed are:

Mass by mass percentage of a solution

⇒ \(=\frac{\text { Mass of solute }}{\text { Mass of solution }} \times 100\)

Mass by volume percentage of a solution

∴ \(=\frac{\text { Mass of solute }}{\text { Volume of solution }} \times 100\)

Question1. A solution contains 50 g of common salt in 450 g of water. Calculate the concentration of the solution.
Answer:

Concentration of solution = \(\frac{\text { Mass of solute }}{\text { Mass of solution }} \times 100\)

Mass of common salt (solute) = 50 g

Mass of water = 450 g

Mass of solution = 50 + 450 = 500 g

Concentration of solution = \(\frac{50}{500} \times 100=10 \%\)

Example 2. 4 g of a solute is dissolved in 40 g of water to form a saturated solution at 25°C. Calculate the solubility of the solute at 25°C.
Answer:

Solubility = \(\frac{\text { Mass of solute }}{\text { Mass of solvent }} \times 100\)

Mass of solute = 4g,

Mass of solvent = 40 g,

Solubility = \(\frac{4(\mathrm{~g})}{40(\mathrm{~g})} \times 100\)

= 10g

Suspension

A suspension is a heterogeneous mixture in which the solute particles do not dissolve, but remain suspended throughout the bulk of the medium, for example., a mixture of chalk powder in water, a mixture of sand in water, smoke coming out of a chimney of a factory.

Properties of Suspension

Some important properties of suspension are as follows:

1. Suspension is a heterogeneous mixture.
2. Its particles can be seen by the naked eye.
3. Its particles scatter a beam of light passing through it and make its path visible (Tyndall effect).
4. It is unstable, i.e. the solute particles settle down when suspension is left undisturbed. They can be separated by the process of filtration. When the particles settle down, the suspension breaks and it does not scatter light any more.

Note: The insoluble particles in a suspension are called ‘suspended particles’, whereas the solvent is referred to as ‘medium’.

Colloidal Solution

A colloid (or colloidal solution) is a mixture that is heterogeneous but appears to be homogeneous as the particles are uniformly spread throughout the solution, for example., milk, shaving cream, cheese, etc. Colloidal solutions are also called colloidal sols.

Properties Of A Colloid: Some important properties of a colloid are as follows :

1. A colloid is a heterogeneous mixture.
2. The size of particles of a colloid is too small to be individually seen by the naked eye.
3. Colloids are big enough to scatter a beam of light passing through it and make its path visible.
4. The colloids are quite stable. Particles do not settle down when a colloid is left undisturbed.
5. Particles of colloid can pass through filter paper, therefore a colloid cannot be separated by filtration. However, they get separated by a special technique called centrifugation.

Common Examples Of Colloids

Colloids are classified according to the state (solid, liquid or gas) of the dispersion medium and the dispersed phase.

Types Of Colloids

Tyndall Effect

The scattering of light by colloidal particles is known as the Tyndall effect. In a true solution, the solute particles are so small that they cannot scatter light falling on them.

• In a colloidal solution, the particles are big enough to scatter light. Tyndall effect can also be observed in the following situations:
• When a fine beam of light enters a room through a small hole (due to scattering of the beam of light by the particles of dust and smoke in the air).
• When sunlight passes through the canopy of a dense forest (as the mist containing tiny droplets of water scatters it). The components of a colloidal solution are the dispersed phase and the dispersion medium.
• The solute-like component or the dispersed particles in a colloid form is the dispersed phase and the component in which the dispersed phase is suspended, is known as the dispersion medium.

Physical And Chemical Changes

Physical Changes

The properties that can be observed and specified like colour, hardness, rigidity, fluidity, density, melting point, boiling point, etc., are physical properties.

• The changes which occur without a change in composition and the chemical nature of the substance are called physical changes.
• The interconversion of states is a physical change, for example., change of water in ice is a physical change because chemically, ice and liquid water both are the same.
• Although ice, water and water vapour all look different and display different physical properties chemically they are the same.

Chemical Changes

In chemical changes, one substance reacts with another substance to change chemical composition. Chemical changes bring a change in the chemical properties of matter and a new substance is obtained.

• A chemical change is also called a chemical reaction, for example., both water and cooking oil are liquid, but their chemical characteristics are different. They differ in odour and inflammability.
• Oil burns in air whereas water extinguishes fire, i.e. it is the chemical property of oil that makes it different from water.

Note:

• Burning is a chemical change.
• During the burning of a candle, both physical and chemical changes take place.

Activity 1

Objective

To prepare homogeneous and heterogeneous mixtures.

Materials Required

Four beakers, copper sulphate powder, common salt, sulphur powder, iron filings, water, spatula, etc.

Procedure

1. Label the beakers as A, B, C and D respectively.
2. In beakers A and B, take 50 mL of water.
3. Now, add one spatula of copper sulphate powder in beaker A and two spatula of copper sulphate powder in beaker B.
4. Stir the solutions and observe the changes.
5. Now, in beakers C and D, take equal quantity of common salt (sodium chloride).
6. In beaker C, add iron filings and in beaker D, add sulphur powder.
7. Mix them with the help of a glass rod and observe the changes.

Observation

In beakers A and B, the composition of the mixture is the same throughout but the intensity of the colour of the solutions is different. In beakers C and Z), the obtained mixture has physically distinct parts and a non-uniform composition.

Conclusion

Beakers A and B have homogeneous mixtures of variable composition. Beakers C and D have heterogeneous mixtures of variable composition.

Question 1. What are heterogeneous mixtures?
Answer: The mixtures in which composition are not uniform throughout and which have physically distinct parts are called heterogeneous mixtures.

Question 2. What name is given to a mixture having a uniform composition and no distinct parts?
Answer: Such mixtures are called homogeneous mixtures.

Question 3. On mixing iron filings with sulphur in a beaker, what type of mixture is obtained?
Answer: Heterogeneous mixture is obtained when iron filings are mixed with sulphur.

Question 4. If the substances present in the beaker as given in Q-3 are heated, what do you observe?
Answer: On heating iron with sulphur, we get black coloured iron
sulphide which is a compound, not a mixture as its properties are quite different from the properties of iron as well as sulphur.

Question 5. Give an example of homogeneous mixture.
Answer: Solution of sugar or salt in water.

Activity 2

Objective

To study the properties of solution, suspension and colloidal solution.

Materials Required

Three beakers, glass rod, copper sulphate crystals, chalk powder (or wheat flour) and few drops of milk or ink.

Procedure

1. Label the beakers as A, B and C.
2. In beaker A, take 20 mL of water and add copper sulphate crystals.
3. In beaker B, take 20 mL of water and add chalk powder (or wheat flour).
4. In beaker Q take 20 mL of water and add few drops of milk or ink.
5. Stir the solutions with glass rod and observe the changes.
6. Now, direct a beam of light from a torch through all the beakers containing the mixtures and observe from the front.
7. Note your observations.
8. Leave all the solutions undisturbed for a few minutes and observe the changes.
9. Filter the solutions and observe whether there is a residue on the filter paper or not.

Observations

1. In beaker A, solution of uniform composition is obtained. It does not affect the beam of light and no change is observed on keeping it undisturbed for a few minutes. Further, there is no residue on the filter paper in this case.
2. In beaker B, solution of non-uniform composition is obtained. It scatters the beam of light, so its path become visible. On keeping undisturbed, the solute, i.e. chalk powder settle down and if we filter it, the chalk powder comes on filter paper as residue.
3. In beaker C, solution of uniform composition is obtained, but it scatters the beam of light and thus, makes the path visible. If left undisturbed, these remain unaffected. Moreover, no residue left over the filter paper.

Conclusion

1. Beaker A contains a true solution. True solutions are homogeneous and stable. They do not show the Tyndall effect and are not separated by filtration.
2. Beaker B contains suspension. Suspensions are heterogeneous, opaque, unstable and exhibit the Tyndall effect. They are separated by filtration.
3. Beaker C contains the colloidal solution. Colloids are also heterogeneous, translucent, stable and exhibit Tyndall effect. They are not separated by filtration.

Question 1. Why is the Tyndall effect not shown by true solutions?
Answer: This is because the particles of true solutions are very small in size (<1 nm) and hence, these are not able to scatter a beam of light.

Question 2. Give a technique through which colloids can be separated.
Answer: Colloids can be separated by centrifugation.

Question 3. Why do colloidal solutions exhibit the Tyndall effect?
Answer: Because the size of their particles is large enough to scatter light.

Question 4. Why filtration technique does not apply to the separation of a true solution?
Answer: Because the pore size of filter paper is much larger than the size of particles of true solutions.

Question 5. Which of the two will scatter light-soap solution or salt solution?
Answer: Soap solution will scatter light because soap solution is a colloid. A salt solution is a true solution so, it will not scatter light.

Activity 3

Objective

To demonstrate that the different substances in a given solvent have different solubilities.

Materials Required

Beakers, salt, sugar/barium chloride, water, burner, glass rod, etc.

Procedure

1. Take approximately 50 mL of water each in two separate beakers.
2. Add common salt in the first beaker and sugar or barium chloride in the second beaker with continuous stirring.
3. Continue the addition till it stops dissolving.
4. When no more solute (salt/barium chloride) can be dissolved, heat the contents of the beakers.
5. Start adding the solute again.
6. Take a saturated solution at a certain temperature and cool it slowly.

Observations

1. When no more salt or barium chloride can be dissolved in a solution or they stop dissolving, a saturated solution of salt or barium chloride is obtained.
2. On heating, when more solute (i.e. salt or barium chloride) is added to the saturated solution, then this solution becomes unsaturated.
3. When a saturated solution at a certain temperature is cooled, the solubility of a solute decreases and the amount of the solute which exceeds the solubility at the lower temperature crystallises out of the solution.

Conclusion

Different substances in a given solvent have different solubilities at the same temperature. In general, the solubility decreases as the solution is cooled and the extra amount crystallises out.

Question 1. What is a saturated solution?
Answer: A solution in which no more solute can be dissolved at any fixed temperature is called a saturated solution.

Question 2. What is an unsaturated solution?
Answer: A solution in which some more solute could be
dissolved at any fixed temperature is called an unsaturated solution.

Question 3. Write the effect of heating on a saturated solution.
Answer: If a saturated solution at a particular temperature is heated
to a higher temperature, then it becomes unsaturated.

Question 4. What happens when a saturated solution is allowed to cool?
Answer: If a saturated solution available at a particular temperature is cooled to a lower temperature, then some of its dissolved solutes will separate in the form of solid crystals.

Question 5. What is a supersaturated solution?
Answer: Any solution containing more solute than the required amount to prepare a saturated solution at any fixed temperature is called a supersaturated solution.

Question 6. How can you convert a saturated solution into an unsaturated solution?
Answer: By adding more solvent or by applying heat.

Summary

A substance that consists of only a single type of constituent particles is called a pure substance.

• An element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions.
• A metal is an element that is malleable, ductile, and sonorous and conducts heat and electricity.
• A non-metal is an element that is neither malleable nor ductile and does not conduct heat and electricity.
• Metalloids are intermediate properties between those of metals and non-metals.
• A compound can be defined as a substance composed of two or more elements, chemically combined in a fixed proportion.
• Mixtures are constituted by more than one substance mixed in any proportion.

Depending upon the nature of the components that form a mixture, we have two types of mixtures:

1. Homogeneous Mixture A mixture in which the constituents are uniformly distributed throughout i.e. without any clear boundary of separation is called a homogeneous mixture.
2. Heterogeneous Mixture A mixture that does not have uniform composition, i.e. has visible boundaries of separation between its constituents is called a heterogeneous mixture.

• A solution is a homogeneous mixture of two or more substances. There are two main components of a solution; solvent and solute.
• The concentration of a solution is the amount of solute present in a given amount (mass or volume) of a solution, or the amount of the solute dissolved in a given mass or volume of the solvent.
• Depending upon the amount of solute present in a given amount of solvent, it can be classified as under:

1. Saturated Solution A solution in which no more amount of solvent can be dissolved at a given temperature, is called saturated solution.
2. Unsaturated Solution If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution.

⇒ \(\text { Concentration of solution }=\frac{\text { Amount of solute }}{\text { Amount of solution }}\)

⇒ \(=\frac{\text { Amount of solute }}{\text { Amount of solvent }}\)

• A suspension is a heterogeneous mixture in which the solute particles do not dissolve, but remain suspended throughout the bulk of the medium.
• A colloid is a mixture that is actually heterogeneous, but appears to be homogeneous as the particles are uniformly spread throughout the solution.
• The scattering of light by colloidal particles is known as Tyndall effect.
• In physical changes, only physical properties of the substance changes.
• In chemical changes, one substance reacts with another substance to undergo a change in chemical composition.

Matter In Our Surroundings Multiple Choice Questions

Question 1. A diver can cut through water in a swimming pool. The property shown by the matter is

1. The particles are of very small size
2. The matter has space between them
3. The particles are in a solid state
4. The particles are running here and there, and have no space between them

Answer: 2. The matter has space between them

A diver can cut through water because water is a liquid and has space between the water molecules due to the weak force of attraction among the water molecules.

Question 2. Crystals of KMnO₄ in water prove that

1. KMnO₄ is of red color
2. KMnO₄ is acidic
3. KMnO₄ is made up of millions of tiny particles
4. KMnO₄ is a reducing agent

Answer: 3. KMnO₄ is made up of millions of tiny particles

When crystals of KMnO₄ are added to water, it dissociates into millions of tiny particles, which proves that KMn04 is made up of millions of tiny particles.

Question 3. Honey dissolves slowly in water as a solvent, because

1. It has a high boiling point
2. It vaporises easily
3. It is highly viscous
4. It has a low melting point

Answer: 3. It is highly viscous

When honey is dissolved in water as a solvent. It dissolves slowly because of its highly viscous nature, i.e. It has high density and a strong force of attraction between the honey molecules.

Question 4. Gases show a faster rate of diffusion because

1. Of higher boiling point
2. Of low intermolecular space
3. Of high melting point
4. Of large intermolecular space

Answer: 4. Of large intermolecular space

Gases show a faster rate of diffusion because they have weak intermolecular forces between them and move at higher speeds. As a result, these have large intermolecular space between the molecules.

Question 5. Which of the following is found in a solid state at room temperature?

1. Stone
2. Sand
3. Mercury
4. Both (1) and (2)

Answer: 4. Both (1) and (2)

• Stone and sand are found in a solid state at room temperature because of a strong attractive force among their particles, (i.e. the particles of stone and sand are held together with strong attractive forces).
• Thus, the intermolecular space among them is very small and these exist as solids, mercury is the only metal which is found in a liquid state at room temperature.

Question 6. The similarity between a liquid and a gas is

1. Both do not show a definite shape
2. Both have a definite volume
3. Both have the same b.p
4. Both have the same nature

Answer: 1. Both do not show a definite shape

• Liquids and gases both are considered fluids but due to weak intermolecular force of attraction, gases occupy the shape and volume of a container.
• Liquids have slightly more force of attraction and thus have definite volume but not definite shape.
• Hence, we can say that gases have no definite volume and shape, (as they occupy the whole volume and shape of a container) but liquids have definite volume but no definite shape (i.e. they occupy the shape of a container.)

Question 7. For a given mass, identify the correct order of density.

1. Water < chalk < cotton <air
2. Chalk < water < cotton < air
3. Air < cotton < water < chalk
4. Cotton < air < water < chalk

Answer: 3 and 4

Mass (m) per unit volume (V) is called density

i.e. \(d=\frac{m}{V}\). As intermolecular space (i.e. volume) among the molecules of the given particles is as follows

chalk < water < cotton < air.

Therefore, the order of density will be,

air < cotton < water < chalk

∴ \(\left[d \propto \frac{1}{V}\right]\)

Hence,(3) is the correct option.

Question 8. A gas fills the vessel in which it is kept, because

1. Of weak intermolecular attractive forces
2. Of strong intermolecular attractive forces
3. Of very weak intermolecular repulsive forces
4. Of fluidity

Answer: 1. Of weak intermolecular attractive forces

A gas fills the vessel in which it is kept because of weak intermolecular attractive forces among the gas molecules. As a result, they can move in the entire vessel and occupy the whole volume of the vessel in which they are kept.

Question 9. A gas can exists the pressure on the walls of the container because

1. Of high molecular mass
2. Of high kinetic energy
3. Of high repulsive forces
4. Of low intermolecular space

Answer: 2.  Of high kinetic energy

Due to the high kinetic energy (K.E.) possessed by the gas particles, they move with high speed and put pressure on the walls of the container.

Question 10. Any substance is said to be a solid because

1. Of definite volume and shape
2. Of high rate of diffusion
3. Of fluidity
4. Of the weak force of attraction

Answer: 1. Of definite volume and shape

A substance is said to be a solid because of its definite volume and shape. Since the particles of solids are rigid and have a strong force of attraction, they offer a definite volume and shape to the solids.

Question 11. A solid substance possess

1. Rigidity fluidity and weak force of attraction
2. Rigidity fluidity and fixed volume
3. Rigidity fixed volume and high attractive forces of attraction
4. Rigidity fixed shape and large intermolecular space between particles

Answer: 3. Rigidity fixed volume and high attractive forces of attraction

• The solids are rigid and have fixed volume due to the strong force of attraction.
• They can not flow and, therefore are not considered as fluid. Due to strong forces of attraction, the intermolecular space between their particles is very small.

Question 12. Add a teaspoon full of table salt to water and stir it for some time. On standing, we find that

1. Water level rises
2. The water level remains as such
3. Water level decreases
4. Water starts boiling

Answer: 2. Water level remains as such

The water level remains as such because table salt dissolves in water completely and occupies the spaces between the particles of water.

Question 13. The same quantity of air and water are compressed with the same pressure, the correct conclusion we find is

1. Air is compressed more than the water
2. Water is compressed more than the air
3. Both are compressed to the same extent
4. No compression is found in the cases

Answer: 1. Air is compressed more than the water

• Air will compressed more than water due to the very weak force of attraction among the air molecules as compared to water. As a result, the intermolecular space in air is more than that of water.
• Hence on applying the same amount of pressure, the air will compressed more than the water.

Question 14. Temperature 200° C equals nearly to

1. 300 K
2. 373 K
3. 473 K
4. -73 K

Answer: 3.

Celsius scale (° C) and Kelvin scale are related as follows.

Temperature on Kelvin scale

= Temperature on ° C + 273.

Hence, temperature on Kelvin scale = 200 + 273 = 473 K

∴ The option is the correct answer.

Question 15. According to the Kelvin scale, 0° C is equal to

1. 0 K
2. 100 K
3. 273 K
4. – 273 K

Answer: 3. 273 K

Question 16. A large volume of compressed natural gas (CNG) is available in small cylinders due to its property of

1. Low density
2. High compressibility
3. Easy availability
4. High inflammability

Answer: 2. High compressibility

Question 17. Which of the following has the highest kinetic energy?

1. Particles of water at 100° C
2. Particles of steam at 100° C
3. Particles of ice at 0° C
4. Particles of water at 0° C

Answer: 2. Particles of steam at 100° C

Question 18. The density of a substance is defined as

1. Product of mass and volume
2. Ratio of mass and volume
3. Product of mass and temperature
4. Ratio of mass and temperature

Answer: 2. Ratio of mass and volume

Question 19. The process involving the change of state from solid to gas state is known as

1. Melting
2. Sublimation
3. Fusion
4. Vapourisation

Answer: 2. Sublimation

Question 20. The temperature at which the solid melts to become a liquid at the atmospheric pressure is known as

1. Melting point
2. Boiling point
3. Freezing point
4. None of these

Answer: 1. Melting point

Question 21. Which of the following is accompanied by cooling?

1. Vaporisation
2. Evaporation
3. Condensation
4. Sublimation

Answer: 2. Evaporation

Question 22. Which of the following does not affect the rate of evaporation?

1. Surface area
2. Insoluble heavy impurities
3. Temperature
4. All of these

Answer: 2. Insoluble heavy impurities

Matter in Our Surroundings Fill In The Blanks.

Question 1. Evaporation of a liquid at room temperature leads to a ____ effect.
Answer:  Cooling

Question 2. At room temperature, the forces of attraction between the particles of solid substances are ____ than those that exist in the gaseous state.
Answer: Stronger

Question 3. The arrangement of particles is less ordered in the state. However, there is no order in the ____ state.
Answer: liquid; gaseous

Question 4. ____ is the change of gaseous state directly to solid state without going through the state.
Answer: Sublimation; liquid

Question 5. The phenomenon of change of a liquid into the gaseous state at any temperature below its boiling point is called ____
Answer: Evaporation

 Class 9 Science Chapter 2 Is Matter Around Us Pure Long Answer Type Questions

Question 1.

1. What are elements?
2. What are the three main types of elements?
3. Write a property of each type of element.

Answer:

1. An element consists of only one type of atom. It is a basic form of matter that cannot be broken down into simpler substances by chemical reactions.
2. Metals, non-metals and metalloids.
3. Metals—Malleable and ductile Non-metals—Brittle Metalloids—Semiconductors

Question 2.

1. Distinguish among the true solution, suspension and colloid in a tabular form under the following heads:
   1. Stability
   2. Filterability
   3. Type of mixture
2. Give expression for the concentration of a solution. How will you prepare a 10% solution of glucose by mass in water?

Answer:

Distinctions between true solution, suspension and colloid are:

The methods by which the concentration of a solution can be expressed are:

Mass by mass% of solution

⇒ \(=\frac{\text { Mass of solute }}{\text { Mass of solution }} \times 100\)

Mass by volume% of solution

⇒ \(\frac{\text { Mass of solute }}{\text { Mass of solution }} \times 100\)

A 10 per cent solution of glucose can be prepared by dissolving 10 g of glucose in 90 g of water.

Read and Learn More Class 9 Science Solutions

Question 3. Write your observations when the following processes take place:

1. An aqueous solution of sugar is heated to dryness.
2. A saturated solution of potassium chloride prepared at 608°C is allowed to cool at room temperature.
3. A mixture of iron filings and sulphur powder is heated strongly.
4. A beam of light is passed through a colloidal solution.
5. Dilute HCl is added to the mixture of iron and sulphur.

Answer:

1. Sugar remains as a residue in the form of a solid mass.
2. Potassium chloride crystallises out.
3. A black-coloured compound is formed.
4. The path of the light becomes visible.
5. A colourless gas has evolved.

Question 4.

1. Pond water contains sand grains, clay particles, salt, pieces of paper and some air bubbles. Select from amongst these, an example each of a solvent, solute, colloid and suspension.
2. Give one example of each of the following:
3. A solution of gas in liquid
4. A solution of two solids
5. A solution of two gases

Answer:

1. Solvent-Water  Solute-Salt, pieces of paper, air bubbles Colloid-Mixture of air bubbles and water Suspension-
   1. A mixture of water and sand grains
   2. A mixture of water and clay particles
2. Aerated drinks,
   1. Brass,
   2. Air

Question 5. Classify each of the following as a physical or a chemical change. Give reasons.

1. Drying of a shirt in the sun.
2. Rising of hot air over a radiator.
3. Burning of kerosene in a lantern.
4. Change the colour of black tea by adding lemon juice to it.
5. Churning of milk cream to get butter.

Answer:

1. Physical change Because the evaporation of water takes place, but no change occurs in the composition of the substance.
2. Physical change It involves ing only the movement of with it, no change in the composition of air.
3. Physical as well as chemical change Physical change occurs when kerosene vapourises. After that, the burning of kerosene is a chemical change because, during burning, kerosene oil gets converted into carbon dioxide and water both of which are new compounds.
4. Chemical change The acid present in lemon juice will react with the constituent (for example., caffeine) present in black tea.
5. Physical change As there is no change in composition. Only the separation of components takes place by the physical phenomenon of centrifugation. However, butter will not change to milk easily.

Question 6. Iron filings and sulphur were mixed and divided into two parts, A and B. Part A was heated strongly while part B was not heated. Dilute hydrochloric acid was added to both the parts and the evolution of gas was seen in both cases. How will you identify the gases that evolved?
Answer:

⇒ \(\underbrace{\mathrm{Fe}+\mathrm{S}}_{\text {Part } A} \stackrel{\Delta}{\longrightarrow} \mathrm{FeS}\)

Part B is not heated, so the reaction will be as such

⇒ \(\underset{\text { (In Part B) }}{\mathrm{Fe}}+2 \mathrm{HCl} \text { (dil.) } \longrightarrow \mathrm{FeCl}_2+\mathrm{H}_2 \uparrow\)

In part A, H₂S gas is produced, which is identified by its characteristic smell of rotten eggs. In part B, H₂ gas is produced.

Hydrogen gas is tested by bringing a burning matchstick near the mouth of the test tube. It burns with a pop sound and water is formed.

Question 7. Rama tested the solubility of four substances at different temperatures and found a gram of each substance dissolved in 100 g of water to form a saturated solution.

1. Which solution is least soluble at 293 K?
2. Which substance shows a maximum change in its solubility when the temperature is raised from 293 K to 313 K?
3. Find the amount of ammonium chloride that will separate when 55 g of its solution at 333 K is cooled to 293 K.
4. What is the effect of temperature on the solubility of a salt?
5. What mass of sodium chloride would be needed to make a saturated solution in 10 g of water at 293 K?

Answer:

1. Potassium nitrate.
2. Potassium nitrate.
3. At 333 K, ammonium chloride dissolved per 100 g = 55g.
   1. At 293 K, ammonium chloride dissolved per 100 g = 37g
   2. When a solution is cooled from 333 K to 293 K, the amount of ammonium chloride that will separate =55 -37= 18g
4. The solubility of a salt (sold) increases with the rise in temperature and vice-versa.
5. At 293K, the amount of NaCl dissolved in 100 g of water = 36 g,

∴ At 293 K, the amount of NaCl dissolved in 10 g water = \(\frac{36}{100} \times 10=3.6 \mathrm{~g}\)

Question 8. During an experiment, the students were asked to prepare a 10% (mass/mass) solution of sugar in water. Ramesh dissolved 10 g of sugar in 100 g of water while Sarika prepared it by dissolving 10 g of sugar in water to make 100 g of the solution.

1. Are the two solutions of the same concentration?
2. Compare the mass % of the two solutions.

Answer:

1. No.
2. Rameshs solution concentration

Mass % = \(\frac{\text { Mass of solute }}{\text { Mass of solution }} \times 100\)

⇒ \(\frac{10 \mathrm{~g}}{(10+100) \mathrm{g}} \times 100\)

⇒ \(\frac{10}{110} \times 100\)

⇒ \(\frac{100}{11}=9.09 \%\)

⇒ \(9.1 \%\)

Sarika s solution concentration

⇒ \(\text { Mass } \%\)

⇒ \(\frac{10}{100} \times 100=10 \%\)

The solution prepared by Ramesh has less percentage by mass than that of Sarika.

Question 9.

1. Under which category of mixtures will you classify alloys and why?
2. Whether a solution is always liquid or not. Comment.
3. Can a solution be heterogeneous?

Answer:

1. Alloys are homogeneous mixtures of metals, or non-metals because
   1. It shows the properties of its constituents, and
   2. It has variable composition, for example., brass is considered a mixture because it shows the properties of its constituents, copper and zinc; and it has a variable composition.
2. A solution is generally a liquid, not always, for example., alloys are known to be solid solutions.
3. The term solution is generally used for ‘true solution. In this case, the solution is always homogeneous.
4. In the case of a colloidal solution, that is not a true solution, the solution is heterogeneous.

Question 10. Pragya tested the solubility of three different substances at different temperatures and collected the data as given below (results are given in the following table, as grams of substance dissolved in 100 g of water to form a saturated solution).

1. What mass of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 50 g of water at 313 K?
2. Pragya makes a saturated solution of potassium chloride in water at 353 K and leaves the solution to cool at room temperature. What would she observe as the solution cools? Explain.
3. Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?
4. What is the effect of a change in temperature on the solubility of a salt?

Answer:

1. At 313 K, solubility of potassium nitrate in 100 g of water = 62

Solubility = \(\frac{\text { Mass of solute }}{\text { Mass of solvent }} \times 100\)

Mass or solute = \(\frac{\text { Solubility } \times \text { Mass of solvent }}{100}\)

∴ Mass of potassium nitrate = \(\frac{62 \times 50}{100}=31 \mathrm{~g}\)

Hence, 31g of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 30 g of water at 313 K.

2. The amount of potassium chloride that should be dissolved in water to make a saturated solution increases with temperature.

Thus, as the solution cools, some of the potassium chloride will precipitate out of the solution and form crystals.

3. The solubility of the salts at 293 K are:

Potassium nitrate 32; Sodium chloride 36, Potassium chloride 33; Ammonium chloride 37 Thus, ammonium chloride has the maximum solubility (37) at 293 K.

4. The solubility of a solid (salt) decreases with a fall in temperature, while it increases with a temperature rise.

 Class 9 Science Chapter 2 Is Matter Around Us Pure Short Answer Type Questions

Question 1. An element is sonorous and highly ductile. Under which category would you classify this element? What other characteristics do you expect the element to possess?
Answer:

This element is categorised as a ‘metal’ because metals are sonorous and ductile. Other properties or characteristics possessed by this element are expected to be the following:

1. They possess metallic lustre.
2. They are good conductors of heat and electricity.
3. They are malleable.
4. They have high tensile strength.
5. They have high densities and high melting points/boiling points too.

Question 2. Classify the following as elements, compounds and mixtures.

1. Gold
2. Marble
3. Air
4. Milk
5. Sugar

Answer:

• Element – Gold
• Compound – Marble and sugar
• Mixture – Air and milk

Question 3. How will you prove that water is a compound?
Answer:

Water is considered to be a compound due to the following reasons:

1. Hydrogen and oxygen are present in a fixed ratio of 1:8 by mass in water.
2. Water has a fixed boiling point, i.e. 100°C at 1 atmospheric pressure.
3. The constituents of water (H and O) cannot be separated from it by simple physical methods.
4. The properties of water are entirely different from those of its constituents, i.e. hydrogen and oxygen.

Question 4. Sucrose (sugar) crystals obtained from sugarcane and beetroot are mixed. Will it be a pure substance or a mixture? Give reasons for the same.
Answer:

It is a pure substance because the chemical composition of sugar crystals is the same irrespective of its source.

Question 5. Define a solution. Give an example of

1. Gas in liquid solution.
2. Gas in gas solution.

Answer:

The solution is a homogeneous mixture of two or more substances, for example., salt in water.

1. CO₂ dissolved in water
2. Air

Question 6. What types of mixtures are represented by the following?

1. Carbon dioxide gas dissolved in water.
2. Air containing suspended particles.
3. Soap bubbles formed by blowing air into the soap solution.
4. Water in milk.

Answer:

1. Homogeneous
2. Heterogeneous
3. Heterogeneous
4. Heterogeneous

Question 7. When egg albumin is added to water the clear solution becomes turbid. How would you test to confirm that it is a colloidal solution?
Answer:

Filter the contents of test tubes. No residue is left on the filter paper, but the filtrate obtained is translucent. Since a colloid cannot be separated by filtration, it is a colloid.

Question 8. Give some examples of the Tyndall effect observed in your surroundings.
Answer:

1. Sunlight passes through the canopy of a dense forest.
2. A fine beam of light enters a room through a small hole.

Question 9. Why does the sky appear blue?
Answer:

So The blue colour of the sky is due to the scattering of light by fine dust particles (i.e. Tyndall effect) which are present in the atmosphere.

Question 10. Give reasons:

1. The path of a beam of light is not visible through a true solution.
2. Particles of suspension can be seen with the naked eye.

Answer:

1. The path of a beam of light is not visible through a true solution because particles of a true solution are not large enough to scatter light.
2. The particle size of the suspension is greater than 100 nm. So, they can be seen with the naked eye.

Question 11. On dissolving chalk powder in water, a suspension is obtained. Give any four reasons to support the fact that the mixture so obtained is a suspension only.
Answer:

So It is supported by the following reasons:

1. White particles of chalk powder can be seen with the naked eye.
2. The particles can be separated by ordinary filter paper.
3. Upon shaking, white turbidity reappears in the solution.
4. Light can be passed through this solution, which suggests that it shows the Tyndall effect.

Question 12. Smoke and fog both are aerosols. In what way are they different?
Answer:

In both smoke and fog, the dispersion medium is the same, i.e. gas, but they differ in the dispersed phase.  In smoke, the dispersed phase is solid while in fog, the dispersed phase is liquid.

Question 13. How do sol and gel differ from each other? Give one example for each.
Answer:

• The sol is a colloid in which tiny solid particles are dispersed in a liquid medium. Examples of sol are ink and soap solution.
• The gel is a semi-solid colloid in which liquid particles are dispersed in a solid. Examples of gel are jellies and gelatin.

Question 14. Classify the following as physical or chemical properties.
Answer:

1. The composition of a sample of steel is 98% iron, 1.5% carbon and 0.5% other elements.
2. Zinc dissolves in hydrochloric acid with the evolution of hydrogen gas.
3. Metallic sodium is soft enough to be cut with a knife.
4. Most metal oxides form alkalis on interacting with water.

Answer:

1. Physical
2. Chemical
3. Physical
4. Chemical

Question 15. On heating, calcium carbonate gets converted into calcium oxide and carbon dioxide.

1. Is this a physical or a chemical change?
2. Can you prepare one acidic or one basic solution by using the products formed in the above process? If so, write the chemical equation involved.

Answer:

The phenomenon given in question is a chemical change because the composition of the product formed is different from the substance taken.

The reaction involved is \(\mathrm{CaCO}_3→{\Delta} \mathrm{CaO}+\mathrm{CO}_2\)

Yes, 1.  \(\mathrm{CaO}+\mathrm{H}_2 \mathrm{O} \longrightarrow \underset{\text { Calcium hydroxide } \\\text { (Basic solution) }}{\mathrm{Ca}(\mathrm{OH})_2}\)

2. \(\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \longrightarrow \underset{\text { Carbonic acid } \\ \text { (Acidic solution) }}{\mathrm{H}_2 \mathrm{CO}_3}\)

Question 16. A child eats chocolate and digests it. In doing so, some physical and chemical changes take place. Identify the changes.
Answer:

Physical change Breaking of chocolate into small pieces.

Chemical change Digestion of chocolate.

Question 17. Explain why particles of colloidal solution do not settle down when left undisturbed, while in the case of a suspension, they do.
Answer:

Colloidal particles are smaller in size. The force of gravity which is acting on colloidal particles is encountered and they do not settle down. However the particles of suspension are larger, so they settle down under the influence of gravity.

Question 18. A solution made by dissolving 50 g of glucose in 250 g of water. Calculate the concentration of this solution in mass percentage.
Answer:

Mass percentage \(=\frac{\text { Mass of glucose }}{\text { Mass of glucose }+ \text { Mass of water }} \times 100\)

⇒ \(\frac{50}{50+250} \times 100\)

⇒ \(\frac{50}{300} \times 100=\frac{50}{3}=16.66 \%\)

Question 19. What volume of ethyl alcohol and water must be mixed to prepare 250 mL of 60% by volume of alcohol in water?
Answer:

Let the volume of ethyl alcohol be x mL

Concentration of solution

⇒ \(\frac{\text { Volume of solvent (ethyl alcohol) }}{\text { Volume of solution }} \times 100\)

⇒ \(60=\frac{x}{250} \times 100\)

⇒ \(x=\frac{250 \times 60}{100}=150 \mathrm{~mL}\)

Question 20. Calculate the mass of potassium sulphate required to prepare its 10 per cent (mass per cent) solution in 100 g of water.
Answer:

∴ \(\text { Mass } \%=\frac{\text { Mass of solute }}{\text { Mass of solution }} \times 100\)

Let the mass of potassium sulphate (solute) = xg

Then, mass of solution = (100 + x) g

⇒ Mass %=10= \(\frac{x}{(x+100)} \times 100\)

⇒ \(1=\frac{x \times 10}{(x+100)} \Rightarrow x+100=10 x\)

⇒ 9x = 100

∴ \(x=\frac{100}{9}=11.1 \mathrm{~g}\)

Question 21. Non-metals are usually poor conductors of heat and electricity. They are non-lustrous, non-sonorous, non-malleable and are coloured.

1. Name a lustrous non-metal.
2. The allotropic form of a non-metal is a good conductor of electricity. Name the allotrope.
3. Name a non-metal which is known to form the largest number of compounds.
4. Name a non-metal other than carbon which shows allotropy
5. Name a non-metal which is required for combustion.
6. Name a non-metal that forms common salt with sodium.

Answer:

1. Iodine
2. Graphite (Carbon)
3. Carbon
4. Phosphorus
5. Oxygen
6. Chlorine

Question 22. Classify the substances given below into elements and compounds.

Answer:

Question 23. Which of the following are not compounds?

1. Chlorine gas
2. Potassium
3. Iron
4. Iron sulphide
5. Aluminium
6. Iodine
7. Carbon
8. Carbon monoxide

Answer:

1. Chlorine gas
2. Potassium
3. Iron
4. Aluminium
5. Iodine
6. Carbon

Substances mentioned above are the elements, not compounds. Cl₂ gas is a molecular element.

Question 24. Classify the following mixtures as homogeneous and heterogeneous.

1. Tincture of iodine
2. Smoke
3. Brass
4. Sugar solution

Answer:

1. Homogeneous
2. Heterogeneous
3. Homogeneous
4. Homogeneous

Question 25. Tell whether each of the following properties describes a homogeneous mixture, a solution, a heterogeneous mixture, a compound or an element.

1. A homogeneous liquid which leaves a solid residue on boiling.
2. A cloudy liquid which after some time appears more cloudy towards the bottom.
3. A colourless liquid which boils at a definite temperature and can be decomposed into simpler substances.

Answer:

1. A solution (a solid like salt dissolved in water from which water evaporates on boiling, leaving a solid residue, salt).
2. A heterogeneous mixture in which suspended particles start settling down at the bottom (muddy water).
3. A compound.

Question 26. What would you observe when

1. A saturated solution of potassium chloride prepared at 60°C is allowed to cool to room temperature.
2. A mixture of iron filings and sulphur powder is heated strongly.

Answer:

1. Crystals of potassium chloride are formed because solubility decreases with a decrease in temperature.
2. The compound iron sulphide is formed.

Answer:

Question 27. Why copper sulphate solution in water does not show the Tyndall effect, but a mixture of water and milk shows?
Answer:

• The solution of copper sulphate in water is a true solution. In a true solution, the solute particles are so small that they cannot scatter light falling on them.
• Hence, copper sulphate solution in water does not show the Tyndall effect.
• A mixture of water and milk is a colloid and in a colloidal solution, the particles are big enough to scatter light. So, the mixture of water and milk shows the Tyndall effect.

Question 28. Determine whether each of the following changes is physical or chemical. Give a reason for your answer.

1. A balloon filled with hydrogen gas explodes upon contact with a spark.
2. Copper turns green on exposure to air and water.

Answer:

1. Chemical change Hydrogen gas burns to form water.
2. Chemical change Copper combines with oxygen from air and water to form a copper oxide which has a green colour.

Question 29. Can physical and chemical changes occur together? Illustrate your answer.
Answer:

• In some cases, physical and chemical changes occur together. One such example is the burning of candles. The solid wax present in the candle first changes into a liquid state and then into a vapour state.
• Both these changes are physical. The wax vapours then combine with oxygen in the air to form a mixture of carbon dioxide and water. This involves a chemical change.
• The unburnt wax vapours again change first to the liquid state and finally to the solid state. This interconversion of states is a physical change. Thus, the burning of a candle involves both physical and chemical changes.

Question 30. Calculate the mass of sodium sulphate required to prepare its 20% (mass per cent) solution in 100 g of water.
Answer:

Mass % of sodium sulphate solution = 20%

Mass of the solvent = 100g

Let the mass of solute (sodium sulphate) = xg

Applying the formula,

⇒ \(\text { Mass } \%=\frac{\text { Mass of solute }}{\text { Mass of solution }} \times 100\)

⇒ \(20=\frac{x \mathrm{~g}}{(x+100) \mathrm{g}} \times 100\)

⇒ 20 (x+ 100) =100x

⇒ 20x + 2000 =100 xH2x

⇒ 100x – 20x = 2000

⇒ 80x = 2000

∴ \(x=\frac{2000}{80}=25 \mathrm{~g}\)

Mass of sodium sulphate = 25 g

Question 31. 110 g of salt is present in 550 g of solution. Calculate the mass percentage of the solution.
Answer:

Mass percentage of solution

⇒ \(=\frac{\text { Mass of solute }}{\text { Mass of solution }} \times 100\)

Mass of solute = 110 g [given]

Mass of solution = 550 g [given]

Mass percentage = \(\frac{110}{550} \times 100\)

= 20 %

Question 32. List the points of difference between homogeneous and heterogeneous mixtures. Or Differentiate between homogeneous and heterogeneous mixtures with examples.
Answer:

The main points of differences between the homogeneous and heterogeneous mixtures are as follows:

Question 33. To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293 K. Find its concentration at this temperature.
Answer:

Mass of solute (NaCl) =36 g

Mass of solvent (H₂O) = 100 g

Mass of solution = 36 +100=136 g

Concentration (by mass) of the solution

⇒ \(\frac{\text { Mass of solute }}{\text { Mass of solution }} \times 100\)

⇒ \(\frac{36}{136} \times 100\)

= 26.47%

Question 36. Classify the following as chemical or physical changes:

• Cutting of trees,
• Melting of butter in a pan,
• Rusting of almirah,
• Boiling of water to form steam,
• The passing of electric current through water and the water breaking down into hydrogen and oxygen gases,
• Dissolving common salt in water,
• Making a fruit salad with raw fruits, and
• Burning of paper and wood.

Answer:

Physical changes

• Cutting of trees,
• Melting of butter in a pan, b
• Oiling of water to form steam,
• Dissolving common salt in water, and
• Making a fruit salad with raw fruits

Chemical changes

• Rusting of almirah
• The passing of electric current through water and breaking down of water into hydrogen and oxygen gases.
• Burning of paper and wood.

Question 35. Classify each of the following as a homogeneous or heterogeneous mixture. Soda water, wood, air, soil, vinegar, filtered tea.
Answer:

Homogeneous mixtures of Air, soda water, vinegar, and filtered tea.

Heterogeneous mixtures of Wood, and soil.

Note: Homogeneous mixtures have the same composition throughout, while the composition of heterogeneous mixtures is not uniform.

Question 36. How would you confirm that a colourless liquid given to you is pure water?
Answer:

• Take a given colourless liquid in a beaker and suspend a thermometer into it. Place the beaker on a wire gauze and start heating with the help of a burner.
• Note down the temperature at which water begins to boil. If the given liquid boils at 100°C, the liquid will be pure water as the boiling point of pure water is 100°C.

Question 37. Which of the following materials fall in the category of pure substance?

1. Ice
2. Milk
3. Iron
4. Hydrochloric acid
5. Calcium oxide
6. Mercury
7. Brick
8. Wood
9. Air

Answer:

Ice, iron, calcium oxide and mercury are pure substances as they have definite composition. Hydrochloric acid is a mixture of hydrogen chloride gas and water, so it is a mixture and not a pure substance.

Question 38. Identify the solutions among the following mixtures.

1. Soil
2. Seawater
3. Air
4. Coal
5. Soda water

Answer:

Sea water, air and soda water, as these are homogeneous mixtures of two or more substances.

Note: Sea water is also considered a heterogeneous solution.

Question 39. Which of the following will show the Tyndall effect?

1. Salt solution
2. Milk
3. Copper sulphate solution
4. Starch solution

Answer:

Milk and starch solution being a colloid will show the Tyndall effect, while salt solution and copper sulphate solution are true solutions that will not show the Tyndall effect due to the small size of their particles.

Question 40. Classify the following into elements, compounds and mixtures.

1. Sodium
2. Soil
3. Sugar solution
4. Silver
5. Calcium carbonate
6. Tin
7. Silicon
8. Coal
9. Air
10. Soap
11. Methane
12. Carbon dioxide
13. Blood

Answer:

Elements Sodium, silver, tin and silicon.

Compounds Calcium carbonate, methane, carbon dioxide and soap.

Mixtures Soil, sugar solution, coal, air, blood and soap. Soap is a compound, but to make it suitable for specific purposes, various substances are added to it. Then, it becomes a mixture.

Question 41. Which of the following are chemical changes?

1. Growth of a plant
2. Rusting of iron
3. Mixing of iron filings and sand
4. Cooking of food
5. Digestion of food
6. Freezing of water
7. Burning of a candle

Answer:

GrowtlTof a plant, rusting of iron, cooking of food, digestion of food and burning of a candle are chemical changes.

Question 42. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
Answer:

Method Of Preparation Of Tea

1. Take some water (solvent) in a pan and heat it.
2. Add some sugar (solute) and boil to dissolve the sugar completely, the obtained homogeneous mixture is called a solution.
3. Add tea leaves (or tea) to the solution and boil the mixture.
4. Now add milk and boil again.
5. Filter the mixture through the tea stainer and collect the filtrate or soluble substances, i.e. tea in a cup. The insoluble tea leaves are left behind as residue in the strainer.

 Class 9 Science Chapter 2 Is Matter Around Us Pure Very Short Answer Type Questions

Question 1. State any one difference between pure and impure substances.
Answer:

The pure substance contains only one kind of particle. All the elements and compounds are pure substances. An impure substance contains two or more different kinds of particles. All the mixtures are impure substances.

Question 2. Identify homogeneous mixtures from the following. Soda water, soil, vinegar, unfiltered tea.
Answer: Soda water and vinegar.

Question 3. Is ice water a homogeneous or heterogeneous substance? Is it a pure or impure substance?
Answer:

Ice water is a heterogeneous, but pure substance as ice is made up of water only, which contains only one kind of particle.

Question 4. The ‘sea water’ can be classified as a homogeneous as well as a heterogeneous mixture. Comment.
Answer:

Sea water is called a homogeneous mixture as it contains dissolved salts in it. It may be called a heterogeneous mixture as it contains various insoluble components too such as sand, microbes, shells made up of calcium carbonate and so many other things.

Question 5. Is fresh air free of dust particles and impurities of all other kinds, a pure substance?
Answer:

No, only elements and compounds are pure substances. Air is a mixture of gases and so, not a pure substance.

Question 6. Give one test to show that brass is a mixture and not a compound.
Answer:

The melting point of brass is not definite. So, it is not a compound but it is an alloy (mixture) of copper and zinc.

Question 7. Tincture of iodine has antiseptic properties. How is it prepared?
Answer: A tincture of iodine is prepared by dissolving iodine in alcohol.

Question 8. What are the two components of a solution?
Answer:

The two components of a solution are:

1. Solute
2. Solvent

Question 9. What is meant by the concentration of a solution?
Answer:

The amount of solute present in a given amount (mass or volume) of solution (or solvent) is known as the concentration of solution.

Question 10. Based on which factor, a solution is said to be dilute, concentrated or saturated?
Answer: Amount of solute present in it.

Question 11. Why particles in a true solution cannot be seen with the naked eye?
Answer: Particles of a true solution are very small in size (less than 1nm), hence they are not visible.

Question 12. The particle size of a substance in water is 200 nm. What is the nature of the solution?
Answer:

As the particle size of a substance is greater than 100 nm, the solution will be a suspension.

Question 13. A student mixes the white of an egg with water and stirs it well. After some time, what did he observe?
Answer: He observed that a cloudy solution is formed.

Question 14. Do suspensions show the property of the Tyndall effect?
Answer:

Yes, suspensions show the property of the Tyndall effect because their particles are too large and scatter the light.

Question 15. State which of the following solutions exhibits the Tyndall effect. Starch solution, sodium chloride solution, tincture of iodine, smoke.
Answer: Starch solution and smoke.

Question 16. ‘Tyndall effect can be observed when sunlight passes through the canopy of dense forest’. Explain, how this occurs.
Answer:

This is because the forest contains mist which in turn contains tiny droplets of water that act as particles of colloid dispersed in air.

Question 17. What are the favourable qualities given to gold when it is alloyed with copper or silver to make ornaments?
Answer:

When alloyed with copper or silver, the gold becomes harder and stronger and its brittleness decreases and becomes suitable for making ornaments.

Question 18. Which of the tubes in Figures (1) and (2) will be more effective as a condenser in the distillation apparatus?

Answer:

Condenser (1) will be more effective in the distillation apparatus because the beads present will provide more surface area for cooling and condensation of the vapour occurs repeatedly by passing through it.

Question 19. How will you justify that the rusting of iron is a chemical change?
Answer:

In a chemical change, a new substance is formed and rust is different from iron. Iron is an element while rust is hydrated oxide of iron (Fe₂O₃  xH₂O). Thus, the formation of rust from iron is a chemical change.

Question 20. Choose the chemical change out of the following. Digestion of food, freezing of water, glowing of electric lamp and a mixture of iron filings with sulphur.
Answer: Digestion of food.

Question 21. What is meant by a substance?
Answer:

A substance which is made up of a single type of particle is called a pure substance, For example., hydrogen, oxygen, sulphur, etc.

Question 22. Try segregating the things around you as pure substances or mixtures.
Answer:

Pure substance- Sugar, common salt, rubber. Mixture- Wood, coal, milk, soap, soil. The soap that we use in our daily lives is a mixture.

Is Matter Around Us Pure Multiple Choice Questions

Question 1. Which of the following is a pure substance?

1. Air
2. Distilled water
3. Steel
4. Brass

Answer: 2. Distilled water

Any substance that contains only one type of particle (for example., molecules/atoms etc.) is said to be a pure substance. Among the given options, distilled water has only one type of particle (i.e. H₂O molecules) while air is a mixture of various gases, steel and brass are alloys.

∴ Hence (2) is the correct option.

Question 2. Which of the following properties does not prove that water is a compound?

1. The ratio of hydrogen and oxygen by mass in water is fixed
2. Water has a fixed boiling point (b.p.)
3. Simple physical methods cannot separate the constituents of water
4. Distilled water and tap water have the same taste and constituents

Answer: 4. Distilled water and tap water have the same taste and constituents

• Distilled water and tap water have different tastes and different constituents.
• Distilled water has pure H₂O molecules while tap water may contain other molecules also.
• The ratio of hydrogen and oxygen by mass remains 1:8 in all the H₂O molecules.
• They have fixed b.p. and cannot be separated into their constituents by simple physical methods.

∴  (4) is the correct option.

Question 3. Two elements X and Y combine to give a product Z. The correct statement about Z is

1. Z has more mass than that of X
2. Z has less mass than that of X
3. Z has less mass than that of Y
4. Z show the same properties as of X and Y

Answer: 1. Z has more mass than that of X

Product Z is formed by the combination of X and Y, thus has more mass than of X and Y separatly. Also Z is entirely a new product, thus cannot show same properties, as of X and Y

Question 4. During the purification of water, which of the following step(s) is incorrect?

1. Sedimentation of water
2. Loading of water with alum
3. Electrolysis of water
4. Chlorination of water

Answer: 3. Electrolysis of water

1. During the purification of water
2. Sedimentation is done to remove suspended particles (for example., solids).
3. Loading with alum is done to remove small particles like clay present in colloidal state.
4. Electrolysis is not done, as it dissociates the water molecules in H₂(g) and O₂(g).
5. Chlorination is done to kill the bacteria’s (e.g use of bleaching powder).

Hence, (3) is the correct option.

Question 5. The pair of substance (s) that can be separated by sublimation is/are

1. NH₄Cl and salt
2. sugar solution in water
3. salt solution in water
4. kerosene oil and water

Answer: 1. NH₄Cl and salt

A mixture of ammonium chloride and salt can be separated by sublimation as NH₄Cl changes to vapour directly, i.e. without changing in liquid state while salt does not.

Hence, (1) is the correct option.

Question 6. To prepare iron-sulphide by heating a mixture of iron filings and sulphur powder, we commonly use

1. Petri-dish
2. Water-glass
3. China-dish
4. Beaker

Answer: 3. China-dish

We commonly use Chinese for heating because it can bear high temperatures on heating the mixture.

Question 7. Which of the following substances cannot be broken down by a chemical method?

1. Ammonia (NH₃)
2. Helium (He)
3. Methane (CH₄)
4. Water (H₂O)

Answer: 2. Helium (He)

Helium cannot be broken down by a chemical method as it is an element, while other substances are compounds and can dissociate in their constituents because ammonia is made by two elements nitrogen and hydrogen, methane is made by carbon and hydrogen and water is made by hydrogen and oxygen.

Question 8. Smoke and fog differ from each other because

1. They have different dispersion medium
2. They have different dispersed phase
3. Both (1) and (2)
4. None of these

Answer: 2. They have different dispersed phase

Smoke and fog both have the same dispersion medium but they differ in dispersed phase. In smoke, the dispersed phase is solid while in fog, the dispersed phase is liquid.

Hence, (2) is the correct option.

Question 9. Which of the following will not show the Tyndall effect?

1. Smoke
2. Foam
3. Jelly
4. Salt solution

Answer: 4. Salt solution

The salt solution will not show the Tyndall effect as it is a true solution and particles of salt solution are very small (size < 1 nm). So, these cannot scatter a beam of light and, hence do not show the Tyndall effect.

Question 10. Which of the following is an example of emulsion?

1. Mist
2. Milk
3. Face cream
4. Both (2) and (3)

Answer: 4. Both (2) and (3)

Milk and face cream, both are emulsions of liquid in liquid. Mist is an aerosol, present in our atmosphere. Hence, is the correct option.

Question 11. Which of the following is a physical change?

1. Rusting of iron
2. Formation of water vapours from water
3. Cooking of food
4. Formation of water from hydrogen and oxygen

Answer: 2. Formation of water vapours from water

Formation of water-vapours from water is a physical change, as we can get back the water by simple physical method (i.e. condensation) while all others are examples of chemical changes because products formed can not give back the former reactant (i.e. substance) by physical methods.

Hence, (2) is the correct option.

Question 12. The correct statement about an article made up of iron getting rusted, is/are

1. The process is called corrosion and is a physical change
2. The process is called dissolution and is a physical change
3. The process is called corrosion and is a chemical change
4. The process is called dissolution and is a chemical change

Answer: 3. The process is called corrosion and is a chemical change

The rusting of an article made up of iron is called corrosion. Corrosion is a chemical change because rust is a chemical compound (product) which has different properties and formula than that of reactants.

∴ \(4 \mathrm{Fe}+3 \mathrm{O}_2+x \cdot \mathrm{H}_2 \mathrm{O} \longrightarrow \underbrace{2 \mathrm{Fe}_2 \mathrm{O}_3 \cdot x \cdot \mathrm{H}_2 \mathrm{O}}_{\text {Rust }}\)

Hence, (3) is the correct option.

Question 13. Which of the following is considered to be a pure substance?

1. Sodium chloride
2. Muddy water
3. Granite
4. Milk of magnesia

Answer: 1. Sodium chloride

Question 14. What is the sulphur?

1. A metal
2. A metalloid
3. A non-metal
4. None of these

Answer: 3. A non-metal

Question 15. Which of the following elements is a non-metal?

1. Fe
2. Na
3. Cu
4. Cl

Answer: 4. Cl

Question 16. Which one of the following is a metalloid?

1. Na
2. Fe
3. Cu
4. As

Answer: 4. As

Question 17. In which of the following the constituents are present in any ratio?

1. Compound
2. Mixture
3. Colloid
4. Solution

Answer: 4. Solution

Question 18. Which of the following is an example of a homogeneous substance?

1. Polluted air
2. Muddy water
3. Salt solution
4. None of the above

Answer: 3. Salt solution

Question 19. Which of the following is not an example of a colloidal solution?

1. Glue
2. Milk
3. Salt solution
4. Blood

Answer: 4. Blood

Question 20. Which one of the following statements is suitable (correct) for the mixture of sulphur and carbon disulphide?

1. Heterogeneous mixture and represent Tyndall effect
2. Homogeneous mixture and represent Tyndall effect
3. Heterogeneous mixture but does not show the Tyndall effect
4. Homogeneous mixture but does not show the Tyndall effect

Answer: 4. Homogeneous mixture but does not show Tyndall effect

Is Matter Around Us Pure Fill In The Blanks

Question 1. A colloid is a mixture and its components can be separated by the technique known as
Answer: Heterogeneous, centrifugation

Question 2. Ice, water and water vapour look different and display different properties, but they are the same.
Answer: Physical, chemically

Atoms And Molecules

In the present chapter, we shall discuss about the various laws (which explains how atoms combine to form molecule), symbols and formulae of atoms and molecules and various ways of expressing their masses.

Laws Of Chemical Combination

Whenever reactants react together to form the products or the elements combine together to form a compound, they do this according to certain laws. These laws are called laws of chemical combination.

Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination which are as follows:

1. Law Of Conservation Of Mass

It states that ‘mass can neither be created nor be destroyed during a chemical reaction.5 This means that in any chemical reaction, the total mass of the reactants is equal to the total mass of the products and there is no change in mass during the chemical reaction.

Question 1. If 4.0 g of sodium carbonate reacts with 10 g of hydrochloric acid, it results in the formation of 2.5 g of carbon dioxide and 11.5 g of sodium chloride solution. Show that these results are by the law of conservation of mass.
Answer:

⇒ \(\underset{(4.0 \mathrm{~g})}{\text { Sodium carbonate }}+\underset{(10.0 \mathrm{~g})}{\text { Hydrochloric acid }} \longrightarrow \underset{(2.5 \mathrm{~g})}{\text { Carbon dioxide }}+\underset{(11.5 \mathrm{~g})}{\text { Sodium chloride }}\)

Here, total mass of reactants = 4.0 + 10 = 14 g

Total mass of products = 2.5 + 11.5 = 14 g

Since the reactants and products have the same mass, this means that there was no loss or gain of mass after the reaction. Hence, the data is in agreement with the law of conservation of mass.

Read and Learn  More Class 9 Science Notes

2. Law Of Constant Proportions/ Law Of Definite Proportions

According to this law, in a chemical substance (or compound), the elements are always present in definite proportions (or ratios) by mass.

For example., In a compound such as water, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8, whatever the source of water. Thus, if 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained.

Similarly, carbon dioxide (CO₂) always contains carbon and oxygen in the ratio of 3: 8. If a sample of CO₂ contains 36 g of carbon then it is compulsory that the sample has 96 g of oxygen.

This is calculated as \(\frac{3}{8}=\frac{36}{x} ;\)

∴ \(x=\frac{36 \times 8}{3}=96 \mathrm{~g}\)

Question 2. Copper oxide was prepared by two different methods. In one case, 1.75 g of the metal gave 2.19 g of oxide. In the second case, 1.14 g of the metal gave 1.43 g of the oxide. Show that the given data illustrate the law of constant proportions.
Answer:

Case 1 Mass of copper = 1.75 g

And a mass of copper oxide = 2.19 g

So, mass of oxygen = Mass of copper oxide – Mass of copper

= 2.19-1.75 =0.44g

Now, in the first sample of the copper oxide compound.

Mass of copper: Mass of oxygen = 1.75: 0.44

⇒ \(\frac{1.75}{0.44}: 1\)

⇒ 3.93:1

Case 2 Mass of copper = 1.14 g

And, a mass of copper oxide = 1.43 g

So, mass of oxygen = Mass of copper oxide – Mass of copper

= 1.43 -1.14 =0.29 g

Now, in the second sample of the copper oxide compound.

Mass of copper: Mass of oxygen = 1.14: 0.29

⇒ \(\frac{1.14}{0.29}: 1\)

393:1 ≈ 4:1

From the above calculations, we can see that the ratio (or proportion) of copper and oxygen elements in the two samples of copper oxide compound is the same, i.e. 4:1. So, the given data verify the law of constant proportions.

Explanation Of Laws Of Chemical Combination: Dalton’s Atomic Theory

Dalton’s atomic theory explained the law of chemical combination. According to Dalton’s atomic theory, all matter (whether an element, a compound or a mixture), is composed of small particles, called atoms.

The Main Postulates Of Dalton’s Atomic Theory

• Every matter is made up of very small particles, called atoms.
• Atoms are indivisible particles which can neither be created nor be destroyed in a chemical reaction.
• Atoms of a given element are identical in mass as well as in chemical properties.
• Atoms of different elements have different masses and chemical properties.
• Atoms combine in the ratio of small whole numbers to form compounds.
• The relative numbers and kinds of atoms are constant in a given compound.

Atoms

Atoms are the smallest particles of an element which may or may not have independent existence but take part in a chemical reaction. These are the building blocks of all matter.

For example., atoms of hydrogen, oxygen, nitrogen etc., are not capable of independent existence whereas atoms of helium, neon etc., are capable of existing independently.

Size Of Atoms

Atoms are very small and their radius is measured in nanometres.

1/10⁹ m = 1 nm or

lm = 10⁹ nm

A hydrogen atom is the smallest atom and its radius is 0.1 nm.

Modern-Day Symbols Of Atoms Of Different Elements

In chemistry, symbols are the representation of an element. It is simple to use the symbol of an element rather than writing a whole word of an element. Dalton was the scientist who introduced symbols for representing elements for the first time.

Symbols for some elements as proposed by Dalton

As Dalton’s symbols for elements were difficult to draw and inconvenient to use, modern symbols for the elements were introduced by J J Berzelius. These are defined as “a shorthand representation of the name of an element”.

• In the beginning, the names of elements were derived from the name of the place where they were found for the first time.
• Nowadays, it is the IUPAC (International Union of Pure and Applied Chemistry) that approves the names and symbols of the elements. Many of the symbols are the first one or two letters of the element’s name in English.
• The first letter of a symbol is always written in capital letters and the second letter is a small letter.

For example., chlorine (Cl), zinc (Zn) and aluminium (Al). Symbols of some elements have been taken from their names in different languages such as Latin, German, Greek etc.

For example.,

• Iron – Fe from Ferrum (Latin name)
• Gold – Au from Aurum (Latin name)
• Potassium – K from Kalium (Latin name)
• Chlorine – Cl from Chloros (Greek name)
• Cobalt – Co from Kobold (German name)
• Sodium – Na from Natrium (Latin name)

Symbols For Some Elements

Atomic Mass

According to Dalton, each element has a characteristic atomic mass. However, determining the mass of an individual atom was a relatively difficult task due to its very small size. Hence, their relative atomic masses were determined using the laws of chemical combinations and the compounds formed.

For this purpose, initially, 1/16 of the mass of an atom of naturally occurring oxygen was taken as a standard unit because of the following two reasons:

1. Oxygen reacted with a large number of elements and formed compounds.
2. This unit gave masses of most of the elements as whole numbers.

However, in 1961, carbon (C-12 isotope) was chosen as a standard reference for measuring atomic masses universally.

Relative Atomic Mass

It is defined as the number of times a given atom is heavier than 1/12th of the mass of 1 atom of carbon-12 (C-12) or it is the average mass of the atom as compared to l/12th the mass of one carbon-12 atom.

Atomic Mass Unit

It is defined as the mass unit equal to exactly 1/12th of the mass of one atom of the C-12 isotope. Earlier, it was abbreviated as amu but according to the latest recommendations of IUPAC, it is now written as ‘u’- unified mass.

Atomic Masses Of Few Elements

Note: Atoms of most of the elements are not able to exist independently. Atoms form molecules and ions. These molecules or ions aggregate in large numbers to form the matter that we can see, feel or touch.

Molecules

The smallest particle of an element or compound which is capable of independent existence and shows all the properties of that substance is called a molecule. In general, a molecule is a group of two or more atoms that are chemically bonded together. Atoms of the same element or different elements can join together to form molecules.

Molecules can be divided into two categories:

1. Molecules Of Elements

The molecules of an element contain the same type of atoms. Molecules of many elements are made up of only one atom of that element, for example., noble gases like argon (Ar), helium (He) etc.

The molecules of most of the non-metals are made up of more than one atom. for example., a molecule of oxygen (O₂) consists of two atoms of oxygen and is known as a diatomic molecule, and ozone (O₃) consists of three atoms of oxygen and is known as a triatomic molecule.

Atomicity

It is defined as the number of atoms present in a molecule. Based on atomicity, molecules can be classified as:

1. Monoatomic molecules consist of only one atom.
   1. For example., He, Ne, Ar, Xe, Fe, Al etc.
2. Diatomic molecules consist of two atoms.
   1. For example., H₂, O₂, N₂, I₂, Br₂, Cl₂, HCl, NaCl etc.
3. Triatomic molecules consist of three atoms,
   1. For example., O₃, CO₂, NO₂ etc.
4. Tetra-atomic molecules consist of four atoms,
   1. For example., P₄, H₂O₂ etc.
5. Polyatomic molecules consist of more than four atoms,
   1. For example., CH₄ (penta-atomic), S₈ (octa-atomic) etc.

Atomicity of Some Elements (Non-metals):

2. Molecules Of Compounds

Atoms of different elements join together in definite proportions to form molecules of compounds.

Prediction Of Number Of Atoms From Mass Ratio

To predict the number of atoms from the mass ratio, divide the given mass of each element by the atomic mass of the element and calculate the simplest ratio between the obtained moles, for example., we know that the mass ratio of nitrogen and hydrogen in an ammonia molecule is 14 : 3. The number of atoms of nitrogen and hydrogen present in the molecule of ammonia can be calculated as,

Thus, in an ammonia molecule, one N and three H-atoms are present hence, the formula of ammonia is NH3.

Ions

When atoms, groups of atoms or molecules lose or gain electron(s) they become charged. These charged species are known as ions. Atoms in solution generally exist in the form of ions. These can be negatively or positively charged and thus can be categorised into two groups.

Cations

The positively charged ions are known as cations, for example., Na⁺, K⁺, Ca²⁺, Al³⁺ etc. These are formed when elements lose electrons. Usually, metals form cations.

Anions

The negatively charged ions are known as anions, for example., Cl^–, Br^–, O^2-, N^3- etc. These are formed when elements gain electrons. Usually, non-metals form anions.

Poly-atomic Ion

A group of atoms carrying charge and acting as a single entity is known as a polyatomic ion. It carries a fixed charge, for example., NO^–₃ (nitrate ion), CO^2-₃ (carbonate ion) and SO^2-₄ (sulphate ion) etc.

Some Ionic Compounds

Note: Ionic compounds are formed by cations and anions, for example., sodium chloride or common salt (NaCl) consisting of a positively charged sodium ion (Na⁺ cation) and negatively charged chloride ion (Cl^– anion).

Valency

The combining power (or capacity) of an element is called its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound. The valency of an ion is equal to the charge of the ion.

Names, Symbols And Valency Of Some Ions

Note These elements show more than one valency. Here, the Roman numeral written in brackets shows their valency.

Writing Chemical Formulae

The shortest way to represent a compound with the help of symbols and the valency of elements is known as a chemical formula. The chemical formula of a compound shows its constituent elements and the number of atoms of each combining element.

In ionic compounds, the charge on each ion is used to determine the chemical formula of a compound.

There are some rules for writing the chemical formula:

1. The valencies or charges on the ion must be balanced.
2. When a compound consists of a metal and a non-metal, the symbol of the metal is written first and on the left whereas of non-metal on its right, for example., calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO) etc., where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium, iron and copper are metals and are written on left.
3. When a compound is formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio, for example., Ca(OH)₂. In case the number of polyatomic ions is one, the bracket is not required, for example., NaOH.

Formulae Of Simple Compounds

To write the chemical formula for simple compounds :

1. Write the symbols of constituent elements and their valencies as shown below.
2. Write the symbol of the cation first followed by the symbol of the anion.
3. Then criss-cross their charges or valencies to get the formula.
4. The positive and negative charges must balance each other and the overall structure must be neutral.

Note The simplest compounds made up of two different elements are also called binary compounds.

Hydrogen sulphide

Note: When the subscript is number 1, subscript is not written.

Carbon tetrachloride

Magnesium chloride

Calcium oxide

Note: When the valency of both elements is numerically equal, the subscripts are not written.

Aluminium oxide

Sodium nitrate

Potassium carbonate

Sodium carbonate

We use brackets when we have two or more of the same polyatomic ions in the formulae, for example.,

Aluminium Hydroxide

Ammonium sulphate

All subscripts must be reduced to the lowest term (except for molecule or covalent compound) for example., Tin oxide

Molecular Mass

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is, therefore, the relative mass of a molecule expressed in atomic mass units (u).

For example., the relative molecular mass of water (H₂O) is 18 u, which can be calculated as

the atomic mass of hydrogen = 1u

the atomic mass of oxygen = 16u

H₂O contains two hydrogen atoms and one oxygen atom.

Therefore, the molecular mass of water is=2 × 1 + 1 × 6 = 18u

Question 3. Calculate the molecular mass of the following substances.

1. Ammonia
2. Hydrochloric acid
3. Phosphorus molecule
4. Hydrogen molecule
5. Oxygen molecule
6. Sulphur dioxide

Answer:

The molecular mass of ammonia (NH₃)

= 1 × 14 + 3 × 1 =17 u

The molecular mass of hydrochloric acid (HCl)

= 1 × 1 + 1 × 35.3

=36.5 u

The molecular mass of phosphorus molecule (P₄)

= 4 × 31

= 124 u

The molecular mass of hydrogen molecule (H₂)

= 2 × 1

= 2u

The molecular mass of oxygen molecule (O₂)

= 2 × 16

= 32 u

The molecular mass of sulphur dioxide (SO₂)

= 32 + 2 × 16

= 64 u

Formula Unit Mass

It is the sum of the atomic masses of all atoms present in a formula unit of a compound. Formula unit mass is calculated in the same manner as we calculate the molecular mass, for example., formula unit mass for sodium chloride (NaCl)

= 1 × 23 +1 × 35.5

= 58.5 u

Activity 1

Objective

To understand that there is no change in mass when a chemical reaction takes place. (To understand the law of conservation of mass experimentally).

Procedure

1. Take any one of the following sets, X and Y of chemicals

1. Copper sulphate, 1.25 g Sodium carbonate, 1.43 g
2. Barium chloride, 1.22 g Sodium sulphate, 1.53 g
3. Lead nitrate, 2.07 g Sodium chloride, 1.17 g

2. Prepare separate solutions of any one pair of substances listed under Xand Yeach in 10 mL water.

3. Take the solution of Y in a conical flask and the solution of X in a small test tube.

4. Hang the test tube in the flask carefully. Put a cork on the flask and weigh it.

In (1), on weighing, its weight is (1.25 + 1.43 + x) g = (2.68 + x)g

In (2), on weighing, its weight is (1.22 + 1.53 + x)g= (2.75 + x)g

In (3), on weighing, its weight is (2.07+ 1.17+ x)g= (3.24 + x)g

x = combined weight of the set-up (conical flask, test tube, cork etc).

5. Now, tilt and swirl the flask, so that the solutions X and F get mixed. We should put a cork on the mouth of the flask so that no gas can pass out if formed.

6. Weigh again.

Observation

The sum of the weights of the products formed is the same as before the mixing of reactants.

In the reaction flask, the following chemical reactions take place (in respective cases):

1. CuSO₄ + Na₂CO₃ → Na₂SO₄ + CuCO₃
2. BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl
3. Pb(NO₃)₂ + 2NaCl → 2NaNO₃ + PbCl₂

Conclusion

The combined mass of the flask and its contents does not change which means mass remains conserved in the reaction. i.e. mass can neither be created nor be destroyed in chemical reactions.

Question 1. Which law is verified by this activity?
Answer: The law of conservation of mass is verified by this activity.

Question 2. When 20 g of BaCl₂ is mixed with 10.6 g of Na₂SO₄, it produces 8.2 g of NaCI and some amount of BaSO₄. What is the mass of BaSO₄ produced?
Answer:

Total mass of reactants = 20+ 10.6

= 30.6 g

So, a mass of products should be 30.6 g

∴ Mass of BaSO₄ = (30.6 – 8.2)

= 22.4 g

Question 3. What role does this law play in the balancing of chemical equations?
Answer: Chemical equations are balanced to satisfy this law.

Question 4. Who gave the law of conservation of mass in chemical reactions?
Answer: Antoine Lavoisier

Question 5. Matter can neither be created nor be destroyed is the law of…
Answer: Matter can neither be created nor be destroyed is the law of conservation of mass.

Activity 2

Objective

To understand how atoms of different elements join together in definite proportion to form molecules of compounds.

Procedure

1. The ratio by number of atoms for a water molecule can be found as follows:

Thus, the ratio by number of atoms for water is H: O = 2: 1

2. The ratio by number of atoms for ammonia molecule can be found as follows:

Thus, the ratio by number of atoms for ammonia is N: H = 1:3

3. The ratio by number of atoms for carbon dioxide molecule can be found as follows:

Thus, the ratio by number of atoms for carbon dioxide molecule is C: O = 1: 2.

Question 1. What is the atomic mass of nitrogen?
Answer: The atomic mass of nitrogen is 14.

Question 2. What is the ratio of N and H by mass in ammonia molecules?
Answer: The ratio of N and H by mass in ammonia molecule is 14 : 3.

Question 3. Find the ratio of C and O by mass in CO₂.
Answer: The ratio of C and O by mass in CO₂ is 3: 8.

Question 4. What is the simplest ratio by several atoms of C and O for CO₂?
Answer: The simplest ratio for CO₂ is 1: 2.

Question 5. Write the molecular formula of ammonia.
Answer: Ammonia is NH₃

Atoms And Molecules Summary

Antoine L Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination which are:

1. Law of Conservation of Mass It states that “mass can neither be created nor destroyed in a chemical reaction”.
2. Law of Constant Proportion It states that “in a pure chemical substance, the elements are always present in definite proportions by mass”. It was given by J Proust and is also known as the law of definite proportions.

• Dalton’s Atomic Theory
  1. It states that matter is made up of very small indivisible particles called atoms.
  2. Atoms of a given element are identical but those of different elements have different masses and chemical properties.
  3. The major drawback of this theory is that atoms are no longer considered indivisible. Discoveries show that atoms are made up of electrons, protons and neutrons.
• Atom It is the smallest particle of matter which takes part in a chemical reaction.
• Symbols of elements are derived from one or two letters of the names of the elements in English, Greek, Latin, German etc. First letter is written in capital and the second one is small, for example., iron: Fe (from Ferrum).
• Relative Atomic Mass It is defined as the number of times a given atom is heavier than 1/12th of the mass of 1 atom of carbon-12.
• Atomic mass unit (amu) now called unified mass (u) is defined as the mass of l/12th of the mass of one atom C-12 isotope.
• Molecule It is an atom or group of bonded atoms that exist independently.
• Molecules of Elements These are made up of atoms of only one kind. for example., Ar, He, O₂, O₃, etc.
• Molecules of Compounds These are made up of atoms of different elements joined together in fixed ratios, for example., H₂O, CH₄, etc.
• Atomicity It is defined as the number of atoms present in a molecule of an element or a compound. Monoatomic (He, Ne etc.) diatomic (H₂, HCl etc.) triatomic (O₃, H₂Oetc.) and polyatomic (P₄, S₈ etc.) molecules consist of one, two, three and more than three atoms respectively.
• Ions are the charged species and can be positively or negatively charged. Positive charged ions are called cations (for example Na⁺, K⁺ etc) and negative charged ions are called anions (for example., Cl^–, O^2- etc). Polyatomic ions consist of a group of atoms that carry a net charge on them. (for example., OH^–, SO^2-₄ etc).
• Ionic Compounds These compounds are made up of cations and anions, for example., NaCl (Na⁺, Cl^–).
• Valency It is the combining capacity of an element and it is equal to charge in the case of ions.
• Molecular Mass It is the sum of atomic masses of all the atoms present in a molecule. It is expressed in atomic mass unit (u). For example., H₂O = 2 × 1 + 16 = 18 u

Atoms And Molecules Multiple Choice Questions

Question 1. A sample of pure water, irrespective of its source, contains 11.1% hydrogen and 88.9% oxygen. The data supports

1. Law of multiple proportions
2. Law of reciprocal proportions
3. Law of constant proportions
4. Law of conservation of mass

Answer: 3. law of constant proportions

Water obtained from any source contains hydrogen and oxygen in the same proportion by mass. Hence, the data supports the law of constant proportion.

Question 2. If 12 g of C is burnt in the presence of 32 g of O₂, how much CO₂ will be formed?

1. 40 g CO₂
2. 44 g CO₂
3. 30 g CO₂
4. 22 g CO₂

Answer: 2. 44 g CO₂

CO₂ is formed by the following reaction.

⇒ \(\underset{12 \mathrm{~g}}{\mathrm{C}}+\underset{32 \mathrm{~g}}{\mathrm{O}_2} \longrightarrow \mathrm{CO}_2\)

By the law of conservation of mass,

Mass of reactants = Mass of products

Mass of C+ Mass of O₂ = Mass of CO₂

12 g + 32 g = 44 g

Therefore, 44 g of CO₂ will be formed.

Question 3. The formula of chloride of a metal M is MCl₃, then the formula of the phosphate of metal M will be

1. M₂PO₄
2. MPO₄
3. M₂(PO₄)₃
4. M₃PO₄

Answer: 2. MPO4

The valency of M is +3

∴ The formula of phosphate of metal M will be MPO₄.

Question 4. How many moles are present in 11.5 g of sodium?

1. 0.5 mol
2. 5.0 mol
3. 0.05 mol
4. 1.5 mol

Answer: 1. 0.5 mol

23 g of Na present in = 1 mol

11.5 g of Na present = \(\frac{1}{23} \times 11.5\)

= 0.5 mol.

Question 5. Which one of the following statements is true?

1. Mass of 0.5 mole of N₂ gas > mass of 0.5 mole of N atoms
2. Mass of 0.5 mole of N₂ gas =mass of 0.5 mole of O₂ gas
3. Mass of 0.5 mole of N₂ gas < mass of 0.5 mole of N atoms
4. Mass of 0.5 mole of N₂ gas = mass of 0.5 mole of N atoms

Answer: 1. Mass of 0.5 mole of N₂ gas > mass of 0.5 mole of N atoms

1. 1 mole of N₂ gas has mass = 28 g
2. 0.5 mole of N₂ gas has mass = 28 × 0.5 =14 g
3. 1 mole of N atoms have mass = 14 g
4. 0.5 mole of N atoms have mass =14 × 0.5 = 7g

∴ Mass of 0.5 mole of N₂ gas > mass of 0.5 mole of N atoms.

Question 6. Which of the following represents 1 amu?

1. Mass of hydrogen molecule 1
2. \(\frac{1}{12}\)th of mass of C-12 atom 12
3. Mass of 0-12 atom
4. Mass of C-12 atom

Answer: 2. \(\frac{1}{12}\)th of mass of C-12 atom 12

Question 7. A 0.24 g sample of a compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen.

What will be the percentage composition of the compound by weight?

1. Boron = 40%, oxygen = 60%
2. Boron = 60%, oxygen = 40%
3. Boron = 35%, oxygen = 55%
4. Boron = 20%, oxygen = 30%

Answer: 1. Boron = 40%, oxygen = 60%

We know that the mass % of any element in a compound

⇒ \(\frac{\text { mass of element }}{\text { mass of compound }} \times 100\)

⇒ \(\text { Mass } \% \text { of boron }=\frac{0.096}{0.24} \times 100=40 \%\)

⇒ \(\text { Mass } \% \text { of oxygen }=\frac{0.144}{0.24} \times 100=60 \%\)

Question 8. Calculate the molecular mass of Al₂(SO₄)₃.

1. 342 u
2. 242 u
3. 249.5 u
4. 288 u

Answer: 1. 342 u

The molecular mass of Al, (SO₄)₃

= 2 × Atomic mass of Al + 3 [Atomic mass of S + 4 × (Atomic mass of O)]

= 2 × 27. 0 u + 3 × (32. 0 u + 4 × 16.0 u)

= 54 u + 3(32 + 64)u = 54 + 3 × 96u

= 54 + 288 u =342u

Question 9. What will be the mass of 5 mole of SO₂?

1. 310 g
2. 320 g
3. 290 g
4. 120 g

Answer: 2. 320 g

Molecular mass of SO₂ = 64 g

⇒ \(\text { Number of mole }=\frac{\text { Mass of } \mathrm{SO}_2(\mathrm{~g})}{\text { Molecular mass of } \mathrm{SO}_2(\mathrm{~g})}\)

Mass of SO₂ (g) = number of mole x molecular mass of SO₂

= 5 × 64

= 320 g

Question 10. Calculate the mass in g of 2NA molecules of CO₂,

1. 88g
2. 22g
3. 44g
4. None of these

Answer: 1. 88g

NA molecules of CO₂ have molecular mass =44 g.

∴ 2 N A molecules of CO₂ have a molecular mass

= 44 x 2 = 88 g

Question 11. How many molecules are in 5.23 g of glucose (C₆H₁₄O₆)?

1. 1.75 x 10²¹
2. 1.75 x 10²²
3. 1.65 x 10²²
4. None of the above

Answer: 2. 1.75 x 10²²

180g glucose has =NA molecules.

∴ 5.23 g glucose has = \(\frac{5.23 \times 6.022 \times 10^{23}}{180}\)

= 1.75 x 10²² molecules.

Question 12. “Mass can neither be created nor destroyed in a chemical reaction” is defined

1. Law of constant proportion
2. Law of conservation of mass
3. Law of reciprocal proportions
4. Law of multiple proportions

Answer: 2. Law of conservation of mass

Question 13. The percentage of copper and oxygen in samples of CuO obtained by different methods was found to be the same. They illustrate the law of

1. Conservation of mass
2. Multiple proportions
3. Constant proportion
4. Reciprocal proportion

Answer: 3. Constant proportion

Question 14. The percentage of hydrogen in H₂O is

1. 11.12
2. 21.12
3. 20.60
4. 80.0

Answer: 1. 11.12

Question 15. The mass of one C atom is

1. 6.022 × 10²³g
2. 12g
3. 6 g
4. 1.99 × 10^-23g

Answer: 4. 1.99 × 10^-23g

Question 16. The correct symbol for silver is

1. Ag
2. Au
3. A1
4. Si

Answer: 1. Ag

Question 17. The chemical symbol P stands for

1. Phosphorus
2. Potassium
3. Polonium
4. Platinum

Answer: 1. Phosphorus

Question 18. Adding electrons to an atom will result in a

1. Anion
2. Cation
3. Molecule
4. Salt

Answer: 3. Molecule

Question 19. One gram of which of the following contains the largest number of oxygen atoms?

1. O₃
2. O
3. O₂
4. All contain same

Answer: 1. O₃

Question 20. How many molecules are present in one gram of hydrogen?

1. 6.02 × 10²³
2. 1.5 × 10²⁴
3. 3.01 × 10²³
4. 4.5 × 10²³

Answer: 3. 3.01 × 10²³

Question 21. Find out the molecular mass of H₂O.

1. 16μ
2. 18μ
3. 24μ
4. 72μ

Answer: 2. 18μ

Question 22. Write down the names of compounds represented by the following formulae.

1. Al₂ (SO₄)₃
2. CaCl₂
3. K₂SO₄
4. KNO₃
5. CaCO₃

Answer:

1. Aluminium sulphate [Al₂(SO₄)₃]
2. Calcium chloride (CaCl₂)
3. Potassium sulphate (K₂SO₄)
4. Potassium nitrate (KNO₃)
5. Calcium carbonate (CaCO₃)

Question 23. What is meant by the term chemical formula?

1. Al₂(SO₄)₃
2. CaCl
3. K2SO₄
4. KNO₃
5. CaCO₃

Answer:

1. OD Aluminium sulphate [Al₂(SO₄)₃]
2. Calcium chloride (CaCl₂)
3. Potassium sulphate (K₂SO₄)
4. Potassium nitrate (KNO₃)
5. Calcium carbonate (CaCO₃)

Atoms And Molecules Fill In The Blanks

Question 1. In a chemical reaction, the sum of the masses of the reactants and products remains unchanged. This is called___
Answer: The law of conservation of mass

Question 2. A group of atoms carrying a fixed charge on them is called___
Answer: Polyatomic ion

Question 3. The formula unit mass of Ca₃(PO₄)₂ is___
Answer: 310 u

Question 4. The formula of sodium carbonate is … and that of ammonium sulphate is ___
Answer: Na₂CO₃; (NH₄)₂SO₄

Class 9 Science Chapter 3 Atoms And Molecules Long Answer Type Questions

Question 1.

1. Explain why the number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas.
2. Explain atomic mass unit.
3. How many atoms are present in
   1. MnO₂ molecule
   2. CO molecule?

Answer:

1. Hydrogen gas exists as a diatomic molecule, i.e. each hydrogen gas molecule (H₂) has two atoms. While helium gas exists as a monoatomic particle that is its atoms exist individually.
   1. Thus, one mole of hydrogen gas has a double number of atoms as compared to one mole of helium gas.
2. The unit of mass equivalent to the twelfth part of the mass of the C-12 isotope of carbon is called atomic mass unit (amu) or unified mass (u).
3. In the MnO₂ molecule, three atoms are present: one manganese atom and two oxygen atoms.
   1. In a CO molecule, two atoms are present: one carbon atom and one oxygen atom.

Question 1. A 0.24 g sample of a compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.
Answer:

Mass of the compound = 0.24 g,

Mass of boron = 0.096 g

Mass of oxygen = 0.l44g

Percentage of boron = \(\frac{\text { Mass of boron }}{\text { Mass of compound }} \times 100\)

⇒ \(=\frac{0.096 \mathrm{~g}}{0.240 \mathrm{~g}} \times 100=40 \%\)

Percentage of oxygen =\(\frac{\text { Mass of oxygen }}{\text { Mass of compound }} \times 100\)

⇒ \(\frac{0.144 \mathrm{~g}}{0.240 \mathrm{~g}} \times 100=60 \%\)

Alternative method

Percentage of oxygen = 100 – a percentage of boron

= 100-40=60%

Read and Learn More Class 9 Science Solutions

Question 2. Calculate the molecular masses of H₂, O₂, Cl₂, CO₂, CH₄, C₂H_6, C₂H₄, NH₃, CH₃OH
Answer:

1. The molecular mass of H₂ (hydrogen)

= Atomic mass of hydrogen × 2 =1 × 2 = 2 u

2. The molecular mass of O₂ (oxygen)

= Atomic mass of oxygen × 2 = 16 × 2 = 32 u

3. The molecular mass of Cl₂ (chlorine)

Atomic mass of chlorine × 2 = 35.5 × 2 = 71 u

4. The molecular mass of CO₂ (carbon dioxide)

= (Atomic mass of carbon x 1) + (Atomic mass of oxygen × 2)

= 12 × 1 + (16 × 2) = 12 + 32 = 44 u

5. The molecular mass of CH₄ (methane)

= (Atomic mass of carbon × 1) +(Atomic mass of hydrogen × 4) =12 × 1 +(1 × 4) =12 +4 = 16 u

6. The molecular mass of C₂H₆ (ethane)

= (Atomic mass of carbon × 2) + (Atomic mass of hydrogen × 6)

= (12 × 2)+ (1 × 6) = 24 + 6 =30 u

7. The molecular mass of C₂H₄ (ethene)

= (Atomic mass of carbon × 2) + (Atomic mass of hydrogen × 4)

= (12 × 2) + (1 × 4) = 24 + 4 = 28 u

8. The molecular mass of NH₃ (ammonia)

= (Atomic mass of nitrogen × 1) + (Atomic mass of hydrogen × 3)

= (14 × 1) + (1 × 3)

= 14 + 3 =17u

9. Molecular mass of CH₃OH (methanol or methyl alcohol) = (Atomic mass of carbon × 1) + (Atomic mass ,of hydrogen × 3) + (Atomic mass of oxygen × 1 ) + (Atomic mass of hydrogen × 1)

= (12 × 1) + (1 × 3) + (16 × 1) + (1 × 1)

= 12 + 3 + 16 + 1

= 32 u

Question 3. Write the formulae for the following and calculate the molecular mass for each one of them.

1. Caustic potash
2. Baking soda
3. Caustic soda
4. Common salt

Answer:

1. KOH

Molecular mass of KOH =39 +16 + 1= 36 u

2. NaHCO₃

Molecular mass of NaHCO₃ = 23+ 1+ 12+ 3 × 16

= 23 + 1 + 12+ 48

=84 u

3. CaCO₃

The molecular mass of CaCO₃

= 40 + 12 + 3 × 16=100 u

4. NaOH

Molecular mass of NaOH =23+16 + 1

=40u

5. C₂H₅OH

Molecular mass of C₃H₅OH or C₂H₆O =2 ×12 + 6 × 1 + 16

=24 + 6 + 16

=46 u

6. NaCl,

Molecular mass of NaCl = 23 + 35.5

= 58.5 u

Question 4. A metal weighing 6 g formed diatomic oxide upon heating in the presence of air. The oxide thus formed weighed 10 g. Write the chemical name of the compound.
Answer:

Suppose the metal is M. The oxide being diatomic, should have the formula as MO.

The amount of oxygen present in the oxide

= 10 -6 = 4g

∵ 32 g O₂ =1 mol

∴ 4 g O₂ =0.125 mol

The reaction for the formation of oxide can be written as

M + O₂ → MO

Upon balancing we get,

M + O₂ → 2MO

1 mole O₂ reacts with 2 moles of metal

∴ 0.125 mole O₂ reacts with 0.25 mole of metal

Hence, we get

0.25 mole of metal = 6 g

or1 mole of metal =24 g

Thus, the metal is magnesium and the oxide is

magnesium oxide (MgO).

Question 5. Find the number of atoms in 120 g of calcium and 120 g of iron. Which one has more atoms and how much is the difference?

[Atomic mass of Ca =40 u, Fe =56 u]

Answer:

Gram molecular mass of Ca = 40 g

∵ 40 g of Ca contains, several atoms

= 6.022 × 10²³

120 g of Ca contains, several atoms

⇒ \(=\frac{6.022 \times 10^{23}}{40} \times 120\)

= 18.066 × 1023

= 1.8066 × 1024 atoms

Again, the gram molecular mass of Fe = 56 g

∵ 56 g of Fe contains several atoms

= 6.022 x 10²³

∴ 120 g of Fe contains several atoms

⇒ \(=\frac{6.022 \times 10^{25}}{56} \times 120\)

= 12.904 x 10²³ = 1.2904 x 1024 atoms

Therefore, 120g of calcium has more number of atoms.

Difference = (1.8066 -1.2904) x 10²⁴

= 5.162 x 10²³ atoms

Question 6. A sample of ethane (C₂H₆) gas has the same mass as 1.5 x 10²⁰ molecules of methane (CH₄ ). How many C₂H₆ molecules does the sample of gas contain?
Answer:

∵ 6.022 x 10²³ molecules of methane have mass

= Gram molecular mass of methane (CH₄)

=12+ 4 × 1 = 16 g

∴ 1.5 x 10²⁰ molecules of methane have mass

⇒ \(=\frac{16 \times 1.5 \times 10^{20}}{6.022 \times 10^{23}}=3.98 \times 10^{-3} \mathrm{~g}\)

This is also the mass of ethane (C₂H₆).

Gram molecular mass of ethane (C₂H₆)

= 2 × 12+ 6 × 1 = 24 + 6

= 30 g As 30 g of ethane has several molecules

=6.022 × 10²³

Question 7. If 3.42 g of sucrose is dissolved in 18 g of water in a beaker, then what will be the number of oxygen atoms in the solution?
Answer:

Step 1 Molar mass of sucrose,

C₁₂H₂₂O₁₁ =12×12+ 1 × 22+ 16 ×11 = 342g/mol

∵ 342 g of sucrose = 1 mole sucrose

∴ 3.42 g ofsucrose \(=\frac{1 \times 3.42}{342}=0.01 \text { mole sucrose }\)

∵ 1 mole of sucrose (C₁₂H₂₂O₁₁) contains O-atoms

= 11× 6.022 × 10²³

∴ 0.01 mole of sucrose will contain O-atoms = 0.01× 11× 6.023×10²³ = 6.6253 × 10²²

Step 2 18 g water (H₂O) = 1 mole O-atoms = 6.022 × 10²³ O-atoms

Step 3 By adding the number of O-atoms present in 3.42 g sucrose and 18 g water, we get 6.022 × 10²³ +6.6253 × 10²²

= 10²²(60.22+ 6.6253)

= 66.844 × 10²²

= 6.68 × 10²² atoms

Question 8. When 3.0 g of carbon is burnt in 8.00 g of oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.0g of oxygen? Which law of chemical combination will govern your answer?
Answer:

First, we find the proportion of mass of carbon and oxygen in carbon dioxide.

In CO₂, C: O =12: 32 or 3: 8

In other words, we can say that

12.00 g carbon reacts with oxygen =32.00 g

∴ 3.00 g carbon will react with oxygen = 8 g

Therefore, 3.00 g of carbon will always react with 8.0g of oxygen to form 11 g of carbon dioxide, even if a large amount (50.00 g) of oxygen is present. This means when 3.00 g of carbon is burnt in 50.00 g of oxygen, only 8.00 g of oxygen will be used to produce.

11.00 g of carbon dioxide. The remaining 42.00 g of oxygen will remain as it is. This reaction will be governed by the law of constant proportions.

Question 9. Which has more number of atoms? 100 g of N₂ or 100 g of NH₃
Answer:

The molar mass of 1 mole of N₂ = 2 × 14= 28 g

∵ 28 g of N₂ has several molecules

=6.022 x 10²³

∴ 100 g of N₂ has several molecules

⇒ \(=\frac{6.022 \times 10^{23} \times 100}{28}=2.1 \times 10^{24}\)

Atoms in 100 g of N₂ = 2.1 x 10²⁴ × 2

= 4.2 × 10²⁴ atoms.

Similarly, the molar mass of 1 mole of

NH₃= 14 + 3X1 =17 g

∵ 17g NH₃ has number of molecules =6.022 × 10²³

∴ 100 g NH₃ has several molecules

⇒ \(=\frac{6.022 \times 10^{24} \times 100}{17}=3.54 \times 10^{24}\)

Atoms in 100 g of

NH₃ =3.54  × 10²⁴ × 4 = 1.416 x10²⁴

Thus, 100 g of NH₃ has more number of atoms.

Class 9 Science Chapter 3 Atoms And Molecules Short Answer Type Questions

Question 1.

1. State the law of constant proportion.
2. In a compound carbon and oxygen react in a ratio of 3:8 by mass to form carbon dioxide. What mass of oxygen is required to react completely with 9 g carbon?

Answer:

1. The law of constant proportion states that “a pure chemical compound always consists of the same elements that are combined in a fixed (or definite) proportion by mass”.
2. Carbon: oxygen (by mass) =3: 8 i.e. 3 g of carbon requires 8 g of oxygen to form carbon dioxide.

∴ 9 g of carbon requires (3 × 8) 24 g of oxygen to form carbon dioxide.

Question 2. State three points of differences between an atom and a molecule.
Answer:

Question 3. Calcium carbonate decomposes on heating to form calcium oxide and carbon dioxide. When 10 g of calcium carbonate is decomposed completely then 5.6 g of calcium oxide is formed. Calculate the mass of carbon dioxide formed. Which law of chemical combination will you use in solving this problem? State the law.
Answer:

The reaction occurs as follows:

According to the law of conservation of mass, Total mass of reactant(s) = Total mass of products(s)

⇒ 10 g = 5.6 g + Mass of CO₂

⇒ Mass of CO₂ =10 —5.6 =4.4g

This problem is solved using the law of conservation of mass according to which mass can neither be created nor be destroyed during a chemical reaction.

Question 4. Give the formulae of the compounds formed from the following sets of elements.

1. Calcium and fluorine 
2. Hydrogen and sulphur
3. Nitrogen and hydrogen
4. Carbon and chlorine

Answer:

Question 5. A vessel containing 28 g of carbon monoxide gas to which 16 g of oxygen gas is added and the mixture is left for a few hours. When checked there was only one kind of gas (carbon dioxide) in the vessel. What change would you observe in the overall mass of the content? Give a reason for your answer.
Answer:

Carbon monoxide (CO) reacts with oxygen (O₂) to form carbon dioxide (CO₂). There will not be any change in the overall mass of the content of the vessel as the mass of the product formed is equal to the mass of reactants.

i.e. Mass of CO₂ =Mass of CO + mass of O₂

= 28+16 = 44 g

The above observation is based on the law of conservation of mass.

Question 6. Write the Latin names of sodium and iron.
Answer:

Latin name ofsodium (Na) = Natrium

Latin name ofiron (Fe) =Ferrum.

Question 7. Which of the following symbols of elements are incorrect? Give their correct symbols.

1. Cobalt (CO)
2. Carbon (c)
3. Aluminium (AL)
4. Helium (He)

Answer:

1. CO is an incorrect symbol of cobalt. Its correct symbol is Co.
2. c is the incorrect symbol of carbon. Its correct symbol is C.
3. AL is an incorrect symbol of aluminium. Its correct symbol is Al.
4. For helium, He is the correct symbol.

Question 8.

1. What is the relative atomic mass of an element?
2. What is the average atomic mass of hydrogen?

Answer:

1. Relative atomic mass is defined as the number of times a given atom is heavier than 1 /12th of the mass of 1 atom of C-12.
2. The atomic mass of hydrogen is taken as lu whereas actually, it is 1.008 u.

Question 9. Define atomic mass unit. State how atoms exist.
Answer:

• The atomic mass unit (amu) is defined as the mass unit equal to exactly 1/12th of the mass of one atom of the C-12 isotope.
• Atoms may exist independently in a free state (For example., He, Ne and Ar) or in a combined state.
• The combined form of atoms may have similar kinds of atoms (H₂, N₂, O₂ etc.) or different kinds of atoms (H₂O, CO₂, H₂S etc).

Question 10. Classify each of the following based on their atomicity.

1. F2
2. NO₂
3. N₂O
4. C₂H₆
5. P₄
6. H₂O₂
7. P₄O₁₀
8. HCl
9. He
10. O₃
11. CH₄
12. Ag

Answer:

1. Monoatomic: Ag, He
2. Diatomic: HCl, F₂
3. Triatomic : NO₂, N₂O, O₃
4. Tetraatomic : P₄, H₂O₂
5. Polyatomic : C₂H₆, P₄O₁₀, CH₄

Question 11. How do molecules react?
Answer:

Molecules exist in the following two forms

1. Molecules of elements These are formed by the combination of two or more atoms of the same element, for example., O₂, H₂, P₄ etc.
2. Molecules of compounds These are formed by the combination of atoms of different elements. For example., NH₃, CO₂ etc.

Question 12. State the number of atoms present in each of the following chemical species.

1. CO^2-₃
2. PO^3-₄
3. P₂O₅
4. CO

Answer:

1. Number of atoms in CO^2-₃

= Number of C-atoms + Number of O-atoms

= 1 + 3 = 4

2. Number of atoms in PO^3-₄

= Number of P-atoms + Number of O-atoms

=1+4=5

3. Number of atoms in P₂O₅

= Number of P-atoms + Number of O-atoms

=2+5=7

4. Number of atoms in CO

= Number of C-atoms + Number of O-atoms =1+1=2

Question 13. Write the cations and anions present (if any) in the following compounds.

1. CH₃COONa
2. NaCl
3. H₂
4. NH₄NO₃

Answer:

Question 14. Give the names of any two elements present in the following compounds: Baking powder, common salt, sulphuric acid
Answer:

• Baking powder (NaHCO₃) Sodium, hydrogen, carbon and oxygen.
• Common salt (NaCl) Sodium and chlorine.
• Sulphuric acid (H₂SO₄) Sulphur, hydrogen and oxygen.

Question 15.

1. Give one word for the following:
   1. Positively charged ion
   2. A group of atoms carrying a charge
2. Mention any two important rules for writing a chemical formula.

Answer:

1. Cation
   1. Polyatomic ion
2. Two rules for writing a chemical formula are:
   1. When a compound consists of a metal and a non-metal, the symbol of the metal is written first and on the left whereas of non-metal on its right.
   2. The valencies or charges on the ions must be balanced in the chemical formula.

Question 16. Write the molecular formulae for the following compounds.

1. Copper (2) bromide
2. Aluminium (3) nitrate
3. Calcium (2) phosphate
4. Iron (3) sulphide

Answer:

Question 17.

1. An element X has a valency of 2. Write the chemical formula for
   1. Bromide of the element
   2. Oxide of the element.
2. Define the formula unit mass of a substance.

Answer:

1. Valency of X = 2

Valency of bromine =1

∴ The formula of compound

= XBr₂

2. Valency of X =2

Valency of oxygen = 2

∴ The formula of compound

= X₂O₂ or XO

Formula unit mass is the sum of atomic masses of all atoms present in a formula unit of a compound. It is calculated by adding the atomic masses of all the atoms present in one formula unit.

Question 18. Write the chemical formulae of the following compound, using the criss-cross method.

1. Magnesium bicarbonate
2. Barium nitrate
3. Answer:

Question 19. Find the ratio by mass of the combining elements in the following compounds.

1. CaCO₃
2. MgCl₂
3. H₂SO₄
4. C₂H₅OH

Answer:

CaCO₃ → Ca : C : O = 40 : 12 : 48

= 10: 3:12

MgCl₂ → Mg : Cl = 24 : 2 × 35.3 = 24 : 71

H₂SO₄ H : S : O = 2 × 1 : 32 : 4 × 16

= 2:32:64 = 1:16:32

C₂H₅OH → C : H : O = 2x 12 : 6x 1 : 16

= 24:6:16 =12:3:8

Question 20. Nitrogen and hydrogen atoms combine in a ratio of 14 : 3 by mass to form an ammonia molecule. Find the formula of the ammonia molecule by calculating the molar ratio. [Given atomic mass of N = 14 u and H = 1 u]
Answer:

Number of nitrogen atoms present in the molecule

⇒ \(=\frac{\text { Proportion by mass }}{\text { Atomic mass }}=\frac{14}{14}=1\)

Number of hydrogen atoms present in the molecule _ Proportion by mass Atomic mass

⇒ \(=\frac{\text { Proportion by mass }}{\text { Atomic mass }}\)

⇒ \(=\frac{3}{1}=3\)

This means many nitrogen and hydrogen atoms combined in ratio =1:3

Thus, the formula of a molecule of ammonia is NH3.

Question 21. Give the chemical formulae for the following compounds and compute the ratio by mass of the combining elements in each one of them.

1. Ammonia
2. Carbon monoxide
3. Hydrogen chloride
4. Aluminium fluoride

Answer:

Question 22. An element Y has a valency of 4, write the formula for its

1. Chloride Sol.
2. Nitrate
3. Chloride
4. Nitrate

Answer:

Question 23. Out of the following which has more atoms?

1. 50 g of Cu,
2. 50 g of Fe

[Atomic mass of Cu = 63.5 u, Fe =56 u]

Answer:

Number of atoms in 63.5 g Cu = 6.022 × 1023

Number of atoms in 1 g \(\mathrm{Cu}=\frac{6.022 \times 10^{23}}{63.5}\)

Number of atoms in 50g Cu

⇒ \(=\frac{6.022 \times 10^{23}}{63.5} \times 50\)

⇒ \(4.74 \times 10^{23} \text { atoms }\)

Number of atoms in 56 g Fe = 6.022 x 1023

Number of atoms in 1 g Fe \(=\frac{6.022 \times 10^{23}}{56}\)

Number of atoms in 50 g Fe \(=\frac{6.022 \times 10^{23}}{56} \times 50\)

⇒ \(=5.37 \times 10^{23} \text { atoms }\)

∴ 50 g of Fe has more number of atoms.

Question 24. A compound XH is formed by the combination of an element X with hydrogen. Find the valency of the element. State the formula of the compound formed by the combination of X with nitrogen and X with oxygen.
Answer:

In the compound XH, the valency of elements X is 1.

1. The valency of nitrogen is 3.

The valency of element X is 1.

∴ The formula of the compound = NX₃

2. The valency of oxygen is 2.

The valency of element X is 1.

∴ The formula of the compound = OX₂.

Question 25. During photosynthesis, a six-carbon atom molecule, glucose (C₆H₁₂O₆), is formed. Calculate its molecular mass.

[Atomic mass C = 12.0uH = 1.0u and O = 16.Ou]

Answer:

The molecular mass of glucose

= 6 × atomic mass of C+ 12 × atomic mass of H + 6 atomic mass of O

= 6 × 12 +12 × 1+6 × 16

= 72 + 12 + 96

=180 u

Question 26. 6 g of coke consisting of 100% carbon, is burnt in the air. Find the number of oxygen atoms consisting of the carbon dioxide gas thus formed.
Answer:

Each carbon atom combines with two oxygen atoms on combustion as follows:

6 g of coke consist \(\frac{6}{12} \times N_A\)

Hence, the number of oxygen atoms

⇒ \(=2 \times \frac{6}{12} \times N_A=N_A\)

⇒ \(\left(N_A=6.022 \times 10^{23} \text { atoms }\right)\)

Question 27. The mass of a single atom of M is 3.05x 10^-22g. What is its atomic mass? What would this element be? Check the periodic table for possible answers.
Answer:

1 mole = atomic mass of an element

= 6.022 x 10²³ atoms

∵ Mass of 1 atom = 3.05 x 10^-22 g

∴ Mass of 6.022 x 10²³ atoms

= 3.05 x 1(T22 x 6.022 x 10²³ =183.671g

This is nearly the atomic mass of tungsten (W^183.8).

Question 28. What are polyatomic ions? Give examples.
Answer:

A group of atoms carrying a charge and behaving like one entity is known as a polyatomic ion. For example., an oxygen atom and hydrogen atom combine to form a hydroxide ion (OH^–) and one C-atom and three O-atoms combine to form a carbonate ion (CO^2-₃).

Question 29. Give the names of the elements present in the following compounds.

1. Quicklime
2. Hydrogen bromide
3. Baking powder
4. Potassium sulphate

Answer:

1. Quicklime Calcium oxide — CaO Elements Calcium and oxygen
2. Hydrogen bromide — HBr Elements Hydrogen and bromine
3. Baking powder Sodium hydrogen carbonate — NaHCO₃ Elements Sodium, hydrogen, carbon and oxygen
4. Potassium sulphate — K₂SO₄ Elements Potassium, sulphur and oxygen.

Question 30. Write the molecular formulae of all the compounds that can be formed by the combination of the following ions.

Cu²⁺, Na⁺, Fe³⁺, Cl^–, SO^2-₄ , PO^3-₄

Answer:

Question 31. In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g of water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.

Sodium carbonate + Ethanoic acid → Sodium ethanoate + Carbon dioxide + Water

Answer:

Total mass of reactants = Mass of sodium carbonate + Mass of ethanoic acid = 5.3 + 6.0 = 11.3 g

Total mass of products =Mass of sodium ethanoate + Mass of carbon dioxide + Mass of water = 8.2 + 2.2 + 0.9 = 11.3 g

Since the sum of masses of reactants is equal to the sum of masses of products, therefore, the observation made is in agreement with the law of conservation of mass.

Question 32. Hydrogen and oxygen combine in a ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Answer:

Since H and O combine in the ratio of 1: 8 by mass.

Therefore, \(\frac{\text { Mass of } \mathrm{H}}{\text { Mass of } \mathrm{O}}=\frac{1}{8}\)

Let the mass of oxygen required to react completely with 3 g of hydrogen gas be x.

⇒ \(\frac{3}{x}=\frac{1}{8}\)

or x = 24 g

Therefore, 24 g of oxygen is required to react with 3 g of hydrogen to form water.

Question 33. Why is it not possible to see an atom with the naked eye?
Answer:

• Atoms are very small, they are smaller than anything we can imagine. More than millions of atoms when stacked would make a layer barely as thick as a sheet of paper.
• These are very small in radii and measured in terms of nanometers (1nm =10^-9m). Hence, it is not possible to see an atom with the naked eye.

Question 34. Write down the formulae of

1. Sodium oxide
2. Aluminium chloride
3. Sodium sulphide
4. Magnesium hydroxide

Answer:

Sodium oxide

Question 35. What is meant by the term chemical formula?
Answer:

It is the shortest way to represent a compound with the help of symbols and valency (charge) of elements, for example., Element

Question 36. How many atoms are present in a

1. H₂S molecule and
2. PO^3-₄ ion?

Answer:

1. In H₂S molecule, three atoms
   1. [i.e. 2 atoms of H + 1 atom of S] are present.
2. InPO^3-₄ ion, five atoms
   1. [i.e. 1 atom of P + 4 atoms of O] are present.

Question 37. Calculate the formula unit masses of ZnO, Na₂O, K₂CO₃. [Given, atomic mass of Zn = 65 u, Na = 23u, K = 39u, C = 12u andO = 16u]
Answer:

Formula unit mass of ZnO (zinc oxide)

= 65 +16 =81 u

Formula unit mass of Na₂O(sodium oxide)

= (23 x 2) + (16 x 1) = 46 + 16 = 62 u

Formula unit mass of K₂CO₃ (potassium carbonate) = (39 x 2) + (12 x 1) + (16 x 3)

= 78+12 + 48 =138 u

Class 9 Science Chapter 3 Atoms And Molecules Very Short Answer Type Questions

Question 1. How did Berzelius assign symbols to the elements?
Answer:

Berzelius assigned symbols to the elements by taking the first one or two letters of the element’s name in English and in some cases the symbols have been taken from the names of elements in different languages such as Latin, German, Greek etc.

Question 2. Give an example of a triatomic molecule of an element.
Answer: Ozone (O₂)

Question 3. Which is octatonic, carbon or sulphur?
Answer: Sulphur (S₈)

Question 4. Write one example of each.

1. Tetra-atomic molecule
2. Diatomic molecule

Answer: Phosphorus (P₄)

Question 5. Is argon monoatomic or diatomic?
Answer: Argon is monoatomic because its atom can exist independently.

Question 6. Give the difference between a cation and an anion.
Answer:

Cation It is a positively charged ion.

For example., Na⁺, K⁺,Ca²⁺,Mg²⁺ etc.

Anion is a negatively charged ion.

For example., Cr^–, Br^–, F^–,O^2-, N^3- etc.

Question 7. What is the difference between a sodium atom and a sodium ion?
Answer:

Question 8. Define a polyatomic ion. Give one example of it.
Answer:

A group of atoms carrying charge in a single entity is called a polyatomic ion. For example., sulphate ion (SO^2-₄) has five atoms.

Question 9. Choose an ionic compound among S₈ , Cu(NO₃)₂ , P₄ , H₂ and O₂?
Answer: Cu(NO₃ )₂ is an ionic compound because it has Cu²⁺ and NO^–₃ ions.

Question 10. What is the molecular formula of aluminium hydroxide?
Answer:

Question 11. What is the chemical formula of ammonium phosphate?
Answer:

Question 12. Choose the correct formulae of sodium sulphide and sodium sulphite: NaS, Na₂SO₄, Na₂S or Na₂SO₃.
Answer: Sodium sulphide = Na₂S Sodium sulphite = Na₂SO₃

Question 13. An element Z has a valency of 3. What is the formula of the oxide of Z?
Answer:

Question 14. If an element X has its valency equal to 3, what will be its formula with carbonate ion?
Answer:

Question 15. Calculate the formula unit mass of Na₂CO₃. [Atomic mass of Na = 23 u, C = 12 u, 0= 16 u]
Answer: Na₂CO₃ =2 × 23+ 12 + 3 × 16=106 u

Question 16. Calculate the formula unit mass of NaHCO₃. [Atomic mass of Na =23 u, H =1 u, C =12 u, O = 16 u]
Answer:

NaHCO₃ – (Atomic mass of Na) + (Atomic mass of H) + (Atomic mass of C) + (3 × Atomic mass of O)

= (23 + 1 + 12 + 3 × 16)

= 84 u

Question 17. What is the mass of sodium in 58.5 g of NaCl?
Answer:

23g [∵ Mass of sodium = 58.5— mass of Cl =58.5-35.5 = 23]

Question 18. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Answer:

‘The relative number and kinds of atoms are constant in a given compound This postulate explains the law of definite proportions.

Question 19. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Answer:

The postulate which is the result of the law of conservation of mass is “atoms are indivisible particles, which can neither be created nor be destroyed in a chemical reaction”.

Question 20. Define atomic mass unit.
Answer:

One atomic mass unit (u) is the mass unit equal to exactly 1/12th of the mass of one atom of the C-12 isotope.

Class 9 Science Chapter 1 Matter In Our Surroundings Long Answer Type Questions

Question 1. What are the differences between solid, liquid and gaseous states?
Answer:

Differences between solid, liquid and gaseous states are as follows

Question 2.

1. Explain the term density. Arrange different states of matter in increasing order of density.
2. Explain how ice floats on water.

Answer:

1. The mass per unit volume of a substance is called density. Density depends upon the volume of the substance. Substances with small intermolecular spaces have small volumes and high densities.
   1. The increasing order of intermolecular spaces between the different states of matter is solid < liquid < gas. Thus, the increasing order for their density would be gas <liquid<solid.
2. Ice is a solid, but it has a cage-like structure in which some spaces are present between the particles of water.
   1. These spaces are trapped by the air particles. These spaces are larger as compared to the spaces present between the particles of water.
   2. Thus, the volume of ice is greater than that of water. Hence, the density of ice is less than water and it floats on water.

Read and Learn More Class 9 Science Solutions

Question 3. Benzoic acid is used as a food preservative. The given graph shows the heating curve for benzoic acid. Study the graph and answer the following questions:

1. At what time does benzoic acid begin to
   1. Melt?
   2. Boil?
2. What is the melting point of benzoic acid?
3. What happens to the temperature while benzoic acid melts?
4. What is the physical state of benzoic acid during a time interval of 35-45 minutes?

Answer:

1. 1. Benzoic acid begins to melt at 20 min.
   2. Benzoic acid begins to boil at 52 min.
2. The melting point of benzoic acid is120°C.
3. The temperature remains constant atl20°C until all the benzoic acid has melted.
4. The physical state of benzoic acid is liquid during the time interval of 35-45 min.

Question 4.

1. What is matter? Write two properties of solids and two properties of liquids.
2. Give reasons for the following:
   1. Ice at 0°C appears colder in the mouth than water at 0°C.
   2. Doctors advise putting strips of wet cloth on the forehead of a person with having high temperature.

Answer:

1. Matter is a substance which has mass and occupies space.
   1. In solids, the force of attraction between particles is strongest and intermolecular space is much less.
   2. In liquids, the force of attraction is relatively less and intermolecular space is more in comparison to solids.
   1. As ice absorbs the latent heat of fusion too from the mouth so, it feels colder than the water at the same temperature, i.e. 0°C.
   2. As the temperature of the patient’s body is high, the water from the wet strips evaporates by absorbing the heat from the body. This lowers the body temperature of the patient. That is why, doctors advised putting strips of wet cloth on the forehead of a person suffering from high fever.

Question 5. The temperature-time graph given below shows the heating curve for pure wax.

From the graph, answer the following:

1. What is the physical state of the substance at points A, B, C, and D?
2. What is the melting point of the wax?
3. What is its boiling point?
4. Which portions of the graph indicate that a change of state is taking place?
5. Name the terms used for heat absorbed during the change of states involved in the above processes.

Answer:

1. At point A = Solid wax
   1. At point B=Solid as well as liquid wax (Melting continues)
   2. At point C=Liquid wax
   3. At point D = Liquid as well as vapour state (Boiling continues)
2. Melting point of wax = 15°C
3. Boiling point of wax = 110°C
4. Straight lines in the curve parallel to the time-axis (tf-axis) indicate the change of state. These portions are Ax to Bx (solid to liquid) and D₁ to D₂(liquid to vapour state).
5. During melting, at the melting point, the heat absorbed is called latent heat of fusion. During boiling, at boiling point, the heat absorbed is called latent heat of vapourisation.

Class 9 Science Chapter 1 Matter In Our Surroundings Short Answer Type Questions

Question 1. Which of the following matters? Chair, air, love, smell, hate, almonds, thought, cold, cold drink, smell of perfume.
Answer:

• Anything that occupies space and has mass is called matter. Matter can exist in three physical states, liquid and gas.
• Chair and almonds are solid states of matter. A cold drink is a liquid state of matter. Air and the smell of perfume are gaseous states of matter.

Question 2. Give reasons for the following observation. The smell of hot sizzling food reaches you several metres away, but to get the smell of cold food, you have to go close.
Answer:

• Particles of matter are continuously moving. They possess kinetic energy. As the temperature rises, particles move faster.
• The particles of the aroma of hot food mix with the particles of air and reach us several metres away, but to get an aroma or smell of cold food, we have to go close because the particles that carry the smell of cold food move slower as compared to particles that carry the smell of hot sizzling food.

Question 3. A diver can cut through water in a swimming pool. Which property of matter does this observation show?
Answer:

This observation shows that the particles of matter have spaces between them.

Question 4. What are the characteristics of the particles of matter?
Answer:

Characteristics of particles of matter are as follows :

1. They are very small in size.
2. They move faster in a gaseous state as compared to a solid or liquid state.
3. They diffuse faster at higher temperatures.

Question 5. The mass per unit volume of a substance is called density (Density = Mass/Volume). Arrange the following in the order of increasing density. Air, exhaust from chimneys, honey, water, chalk, cotton and iron.
Answer:

The increasing order of density is : Air < exhaust from chimneys < cotton < water <honey < chalk < iron

Question 6.

1. Tabulate the differences in the characteristics of states of matter.
2. Comment upon the following.

Rigidity, compressibility, fluidity, filling a gas container, shape, kinetic energy and density.

Answer:

1. For differences Refer to the text on the gaseous state and effect of change of temperature on page 4.
2. Rigidity The property due to which an object retains its shape and size is known as rigidity. Solids are rigid while liquids and gases are not. Compressibility

The property due to which a substance is reduced to its lower volume when force is applied is called compressibility. Gases are the most compressible while solids and liquids are not.

• Fluidity The property due to which a substance tends to flow is known as fluidity. Gases and liquids can flow, hence they are known as fluids.
• Filling a gas container Particles of a gas move freely in all directions and occupy all the space available to them. Hence, gas fills the container.
• Shape The geometry of an object is called its shape. Solids have a definite shape while gases and liquids do not.
• Kinetic energy The energy of particles of matter due to their movement is called their kinetic energy.
• Gases have maximum kinetic energy among the three states of matter. Kinetic energy increases with the rise in temperature and vice-versa.

Density The mass per unit volume of a substance is called its density.

∴ \(\text { Density }=\frac{\text { Mass }}{\text { Volume }} \text { or } D=\frac{m}{V}\)

Generally, a substance has a maximum density in its solid state as compared to a liquid or gaseous state. Units of density are kg m-3 or g cm-3.

Question 7. Give reasons.

1. A gas fills the vessel in which it is kept.
2. A gas exerts pressure on the walls of the container.
3. A wooden table should be called a solid.
4. We can easily move our hands in the air but to do the same through a solid block of wood, we need a karate expert.

Answer:

1. Particles of gas have the least forces of attraction between them hence, they move freely in all directions and occupy all the space available to them. Hence, a gas fills the vessel in which it is kept.
2. Due to the high kinetic energy possessed by the gas particles, they randomly move at a high speed within the container.
   Due to this random movement, the particles hit each other as well as the walls of the container. The force by which these particles strike the container exerts pressure on its walls.
3. A wooden table has a definite shape and volume. It is very rigid and cannot be compressed. Wood has all the characteristics of a solid. Hence, a wooden table should be called a solid.
4. Particles of air are very far apart from each other due to negligible forces of attraction between them. Therefore, our hands get sufficient space to move in the air. We also displace the air particles without much effort.
   1. But in a solid block of wood, particles are closely packed with the least space between them due to strong intermolecular forces of attraction. So, there is no possibility of moving hands through a block of wood.

Question 8. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.
Answer:

• The mass per unit volume of a substance is called density. As the volume of a substance increases, its density decreases.
• Though ice is solid, it has a cage-like structure in which some spaces are present between the particles of water (these spaces are left when water solidifies). These spaces are trapped by the air particles.
• In fact, these spaces are larger as compared to the spaces present between the particles of water. Thus, the volume of ice is greater than that of water.
• Hence, the density of ice is less than that of water. A substance with a lower density than water can float on water. Thus, ice floats on Water.

Question 9. Convert the following temperatures to Celsius scale.

1. 300 K
2. 573 K

Answer:

For converting Kelvin to Celsius, the formula is

K-273 = °C

1. 300K-273 =27°C
2. 573 K – 273 = 300°C

Question 10. What is the physical state of water at

1. 250°C?
2. 100°C?

Answer:

1. Water vapour or steam.
2. Liquid water as well as water vapour, as steam and water, co-exist atl00°C.

Question 11. For any substance, why does the temperature remain constant during the change of state?
Answer:

1. During the change of state, the temperature remains constant because the heat provided is utilised for breaking the attraction forces between the particles of the substance.
2. This happens at the melting point (or boiling point) of the substance and the heat used is called the latent heat of fusion (or vapourisation). During condensation or solidification, the vice-versa happens.

Question 12. Suggest a method to liquefy atmospheric gases.
Answer:

Applying high pressure and reducing temperature, helps to liquefy atmospheric (or any other) gases. Because under these conditions, the particles come closer, kinetic energy decreases and the gas is liquefied.

Question 13. Why does a desert cooler cool better on a hot dry day?
Answer:

• On a hot dry day, the temperature is high and the humidity is low. The rate of evaporation increases with an increase in temperature and a decrease in humidity. A desert cooler functions on the principle of evaporation.
• The water takes heat from the hot desert cooler and evaporates. The evaporation of water cools the pads and the circulating water. As a result, the incoming air also gets cooled down.

Question 14. How does the water kept in an earthen pot (matka) become cool during summer?
Answer:

• Earthen pots contain tiny pores. During summer, when water is poured into an earthen pot, some of the water seeps through pores to the outer surface.
• The water molecules on evaporation escape from the tiny pores of the earthen pot. The heat required for evaporation is taken from the earthen pot and the water in it.
• This results in a lowering of the heat content of the remaining water and the water becomes cool.

Question 15. Why does our palm feel cold when we put some acetone petrol or perfume on it?
Answer:

Acetone or petrol are volatile liquids which evaporate readily. When these liquids are kept on the palm, their particles gain energy from the palm or surroundings and evaporate, thus causing the palm to cool.

Question 16. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?
Answer:

A saucer or plate has more surface area in comparison to a cup. Therefore, the evaporation of tea occurs more in the saucer rather than cup and more cooling is observed in a saucer.

Question 17. What type of clothes should we wear in summer?
Answer:

We should wear cotton clothes in summer because cotton is a good absorber of water and helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation thereby, causing a cooling sensation.

Question 18. Why is ice at 273 K more effective in cooling than water at the same temperature?
Answer:

When ice melts, it absorbs the energy equal to the latent heat of fusion from the surroundings so, it causes cooling more effectively than the water at the same temperature (because water does not absorb energy from the surroundings).

Question 19. What produces more severe burns, boiling water or steam?
Answer:

Steam causes more severe burns than boiling water. The reason is that it releases the extra amount of heat (latent heat) which it has already taken during vapourisation (when the steam was formed from water).

Question 20. Name A, B, C, D, E and F in the following diagram showing change in its state.

Answer:

A = Melting or fusion, here the solid changes into a liquid.

B = Evaporation or vapourisation, here the liquid changes into a gas.

C = Condensation or liquefication, here the gas changes into liquid.

D = Freezing or solidification, here the liquid changes into a solid.

E Sublimation, here solid directly changes into a gas without coming into a liquid state.

F-Sublimation, here gas changes into a solid without coming in a liquid state.

Question 21. Convert the following temperatures to the Celsius scale.

1. 293 K
2. 470 K

Answer:

1. 293 K- 273 =20°C
2. 470K-273 = 197°C

Question 22. Convert the following temperatures to the Kelvin scale.

1. 25°C
2. 373°C

Answer:

1. 23°C + 273 =298 K
2. 373°C + 273 = 646 K

Question 23. Give a reason for the following observations:

1. Naphthalene balls disappear with time without leaving any solid.
2. We can get the smell of perfume sitting several metres away.

Answer:

1. Naphthalene being a sublimable substance converts directly from a solid to a gaseous state by taking heat from the surroundings through the process, called sublimation.
   1. The naphthalene balls keep on forming naphthalene vapours which slowly disappear into the air. Hence, no residue is left after some time.
2. The smell or aroma of perfume reaches several metres away due to the fast diffusion of the gaseous particles of perfume through air.

Question 24. Arrange the following substances in increasing order of forces of attraction between the particles-water, sugar, and oxygen.
Answer: Oxygen (gas) < Water (liquid) < Sugar (solid).

Question 25. What is the physical state of water at

1. 25°C?
2. 0°C?
3. 100° C?

Answer:

1. Liquid state
2. Solid or liquid state (transition state)
3. Liquid or gaseous state

Question 26. Give two reasons to justify.

1. Water at room temperature is a liquid.
2. An iron almirah is solid at room temperature.

Answer:

1. Water is liquid at room temperature as
   1. It tends to flow.
   2. It takes the shape of the vessel in which it is filled, but its volume does not change.
2. An iron almirah is solid at room temperature because
   1. It has a definite shape and volume.
   2. It is hard and rigid.

Question 27. Carbon dioxide (CO2) is a gas. Justify the given statement by giving two reasons.
Answer:

The two reasons to justify that carbon dioxide is a gas are:

1. Carbon dioxide does not have a fixed volume. It can be compressed by applying pressure.
2. Carbon dioxide does not have a fixed shape. It can take the shape of the container in which it is filled.

Question 28. Give one similarity and one dissimilarity between a liquid and a gas.
Answer:

Similarity Both liquids and gases are fluids and take the shape of the container in which they are placed.

Dissimilarity A gas can be compressed easily whereas a liquid cannot. A small quantity of gas can fill an entire given container. Also large quantity of a gas can be contained in a small space. A given amount of a liquid has a fixed volume at a given temperature.

Question 29. How melting point of a substance indicate the strength of its forces of attraction?
Answer:

More stronger the force of attraction, the more energy is required to break these forces. Hence, the substance which has a higher melting point indicates that its particles are tightly held.

Question 30. The melting points of two solids X and Y are 300 K and 400 K, respectively. Which has more intermolecular forces?
Answer:

The melting point of a solid is an indication of the strength of intermolecular forces between the particles of the solid. It shows that the strength of forces would be maximum in Y and minimum in X.

Question 31. Interconversion of states of matter occurs at constant temperatures. Explain.
Answer:

During the interconversion of state from solid to liquid or from liquid to gas, the temperature remains constant till all the solid has melted or all the liquid has vapourised. The heat energy is used up to overcome the forces of attraction.

Question 32. It is not proper to regard the gaseous state of ammonia as vapours. Explain.
Answer:

The gaseous state of a substance can be regarded as vapours only in case, if it is a liquid or solid at room temperature. Since ammonia is a gas at room temperature, its gaseous state cannot be regarded as vapours.

Question 33.

1. The conversion of solid to vapours is called sublimation. Name the term used to denote the conversion of vapours to solids,
2. The conversion of a solid state to a liquid state is called fusion. What is meant by the latent heat of fusion?

Answer:

1. It is also known as sublimation.
2. The amount of heat energy required to change 1 kg of a solid into liquid at atmospheric pressure and at its melting point is known as the latent heat of fusion.

Question 34. Alka was making tea in a kettle. Suddenly she felt intense heat from the puff of steam gushing out of the spout of the kettle. She wondered whether the temperature of the steam was higher than that of the water boiling in the kettle. Comment.
Answer:

• The temperature of both boiling water and steam is 100°C, but steam gives out more energy (due to latent heat of vapourisation) in comparison to boiling water.
• It is because, when water changes into steam, it absorbs the latent heat of vapourisation, but when steam condenses to form water, an equal amount of latent heat is given out without changing the temperature.

Question 35. Which phenomenon occurs during the following changes?

1. Formation of clouds
2. Drying of wet clothes
3. Wax melts in the sun
4. The size of naphthalene balls decreases

Answer:

1. Condensation
2. Evaporation
3. Fusion
4. Sublimation

Question 36. How will you change water from a gaseous state to a liquid state? Suggest a simple activity.
Answer:

• Water can be changed from a gaseous state to a liquid state by passing the water vapour through a water condenser as used in the case of simple distillation.
• Activity Take ice-cold water in a glass. Observe the outer surface of the glass. You find small droplets of water on it.
• These water droplets are formed as a result of condensation of water vapour present in the air to form liquid water.

Question 37. How do changes in temperature and humidity affect the rate of evaporation?
Answer:

• The rate of evaporation increases by increasing the temperature of the liquid. When the temperature of a liquid is increased by heating, more particles of the liquid get enough kinetic energy to go into a vapour state. This increases the rate of evaporation.
• When the humidity of the air is low, then the rate of evaporation is high and water evaporates rapidly. When the humidity of air is high then the rate of evaporation is low and water evaporates very slowly.

Question 38. Osmosis is a special kind of diffusion. Comment.
Answer:

• Diffusion is the process in which molecules of a substance move from the place of (their) higher concentration to the place of (their) lower concentration.
• But during osmosis, the water (or solvent) molecules move from (their) lower concentration to the place of their higher concentration through a semipermeable membrane. Thus, osmosis is termed a special kind of diffusion.

Question 39. Why does honey diffuse in water at a slower rate than ink?
Answer:

The density of honey is greater than that of ink, thus, honey takes a longer time to spread and hence, honey diffuses at a slower rate than ink.

Question 40. The table below shows the melting and boiling points of four pure substances. Which substance is a liquid at room temperature and would rapidly evaporate if left exposed to the air?

Question 41.

1. A sponge can be compressed, yet it is solid. Explain.
2. Name the state of matter that has minimum space between particles.

Answer:

1. A sponge has minute holes, in which air is trapped, when we press it, the air is expelled out and we can compress it. Hence, a sponge can be compressed, instead of being a solid.
2. Solid state has minimum space between their particles.

Question 42. The cover plate is removed from the gas jars shown in the diagram. After several days, the colour of the gas is the same in both jars. Why does this happen? Explain.

Answer:

• Diffusion has occurred in the jars. Bromine molecules move from a region where they are of higher concentration to a region of lower concentration, i.e. they move in the above gas jar.
• Oxygen molecules move from a region where they are of higher concentration to a region of lower concentration, i.e. they move to the below gas jar.
• Diffusion continues until both gas jars have a uniform distribution of bromine and oxygen molecules.

Question 43. Classify the following into osmosis/ diffusion.

1. Swelling up of a raisin on keeping in water.
2. Spreading of virus on sneezing.
3. Earthworms die on coming in contact with common salt.
4. Shrinking of grapes kept in thick sugar syrup.
5. Preserving pickles in salt.
6. Spreading of smell of cake being baked throughout the house.
7. Aquatic animals use oxygen dissolved in water during respiration.

Answer:

Osmosis

1. Swelling up of a raisin on keeping in water.

3. Earthworms dying on coming in contact with common salt.

4. Shrinking of grapes kept in thick sugar syrup.

5. Preserve pickles in salt.

Diffusion

2. Spreading of virus on sneezing.

6. Spreading of the smell of cake being baked throughout the house.

7. Aquatic animals use oxygen dissolved in water during respiration.

Question 44.

1. Explain the interconversion of three states of matter in terms of the force of attraction and kinetic energy of the molecules.
2. Arrange the three states of matter in the increasing order of rate of diffusion and particle motion.

Answer:

1. During the interconversion of a solid into a liquid and liquid into a gas at an increasing temperature, the kinetic energy of the molecules increases and the force of attraction among molecules decreases and vice-versa.
   1. Rate of diffusion, Solid < liquid < gas
   2. Particles motion, Solid < liquid < gas.

Question 45. How do you differentiate between solids, liquids and gases based on their melting and boiling points?
Answer:

Solids have melting and boiling points above room temperature. Liquids have a melting point below room temperature and a boiling point above room temperature. Gases have both melting and boiling points below room temperature.

Question 46.

1. Dry ice is compressed under high pressure. What happens to it when the pressure is released?
2. Define
   1. Melting point
   2. Fusion.

Answer:

1. On releasing the pressure, dry ice sublimes to a vapour state without undergoing a liquid state.
   1. Melting point The definite temperature at which a solid starts melting is called the melting point of that solid, for example., the melting point of ice is 0°C or 273.16 K.
   2. Fusion The process of conversion of a solid into liquid state on heating is called fusion or melting.

Question 47. What is dry ice? How is it formed?
Answer:

• Dry ice is solid carbon dioxide which sublimes to form carbon dioxide gas without going into a liquid state. The term dry is used to denote sublimation.
• On sublimation, vapours of carbon dioxide create a foggy dense appearance denoted by the term ice. Hence, solid carbon dioxide is also termed dry ice.
• Dry ice is formed when gaseous carbon dioxide is compressed and stored under high pressure. On decreasing pressure to 1 atm, it again changes to the gaseous form without coming into a liquid state.

Question 48. Draw a well-labelled diagram showing the sublimation of ammonium chloride.
Answer:

Take some ammonium chloride in a China dish and place the China dish on a tripod stand. Cover the China dish with an inverted glass funnel.

• Sublimation of ammonium chloride Put a loose cotton plug in the upper open end of the funnel to prevent the ammonium chloride vapour from escaping into the atmosphere.
• The China dish is heated by using a burner. On heating, ammonium chloride changes into white vapours.
• These vapours rise up and get converted into solid ammonium chloride on coming in contact with the cold, inner walls of the funnel.

Question 49. From the graph given below:

1. Which region contains only solids?
2. Which region contains all liquids?
3. Which region shows latent heat of vapourisation?

Answer:

1. AB is the region that contains only solids.
2. The CD is the region that contains all liquids.
3. DE is the region that shows latent heat of vapourisation.

Question 50. Comment on the following statements:

1. Evaporation produces cooling.
2. The rate of evaporation of an aqueous solution decreases with an increase in humidity.
3. Sponge though compressible is a solid.

Answer:

1. Evaporation of liquid produces cooling because liquid takes away the heat from the surroundings, thereby producing a cooling effect.
2. If humidity is high, then air is already saturated with water vapour, i.e. it has a lot of water vapour. Therefore, it will not take more water vapour easily. Hence, the rate of evaporation decreases.
3. The sponge has minute holes in which air is trapped. The material is also not so rigid. On pressing this, air is expelled out, which is why, it can be compressed, but it is solid as it has a definite shape and volume and does not change its shape unless compressed.

Question 51.

1. How will you show that the process of evaporation depends on the nature of the liquid?
2. Why a drop of Dettol is evenly distributed in a bucket of water without the need to stir?

Answer:

1. Take 10 mL of ether (a low boiling point liquid with a boiling point of 34°C) in a test tube and 10 mL of water (boiling point 100°C) in another test tube. Keep both the test tubes near the window for some time. It is observed that ether evaporates in a shorter time. Thus, the lower the boiling point of the liquid, the higher is its rate of evaporation.
2. A drop of Dettol is diffused in water and can be distributed throughout the water since there is enough space between the particles of water.

Question 52. Look at the following figures and suggest in which of the vessels A, B, C or D, the rate of evaporation will be the highest. Explain.

Answer:

• The rate of evaporation will be highest in vessel C as the surface area exposed for evaporation is larger than B ( smaller size).
• The moving fan increases the wind speed which also increases the rate of evaporation. Although A and D are also equal in size to C, A is at a greater distance from the fan and D is covered with a lid.

Question 53.

1. Which factors determine the state of a substance?
2. Convert 30°C into Kelvin.
3. Water droplets are observed on the outer surface of a glass tumbler containing ice-cold water. Give reason.

Answer:

1. Temperature and pressure determine the state of a substance.
2. 30°C = 273 + 30 = 303 K
3. Water droplets are observed on the glass tumbler because water vapours present in the air get condensed on the cold surface of the glass which appears as water droplets.

Question 54. It is a hot summer day. Priyanshi and Ali are wearing cotton and nylon clothes, respectively. Who do you think would be more comfortable and why?
Answer:

• Priyanshi would be more comfortable. The reason is that cotton absorbs sweat from the body and provides a larger surface area for evaporation which causes more cooling effect.
• Nylon being a bad absorber of water does not absorb sweat. Thus, the sweat does not evaporate from the body and Ali would feel uncomfortable.

Question 55. Tarun got an invitation to attend a party. On coming to the place, he found that both his shirt and pants were wet. What step he would take to dry them quickly?
Answer:

Tarun can take the following steps to dry his clothes quickly:

1. By spreading them in the air under the sun so that the water may evaporate (increasing the surface area).
2. By spreading them under a fan in a room (increasing the wind speed).
3. By ironing the clothes (increasing the temperature).

Question 56. While heating ice in a beaker with a thermometer suspended in it, a student recorded the following observations.

Based on the above observations, answer the following questions:

1. State the change (s) observed between 2-3 min and name the process involved.
2. Between 30-35 min, the temperature remains constant. State the reason for this. Name the heat involved in the process and define it.

Answer:

1. Between 2-3 min, ice is converted into water and this process is called fusion.
2. Between 30-35 min, heat is used for converting water (liquid) to vapour (gas), so the temperature during this period remains constant. It is due to the latent heat of vapourisation.
   1. Latent heat of vapourisation The amount of energy required to change 1 kg of water to its vapours at its boiling point is known as the latent heat of vapourisation.

Question 57.

What temperature on the Kelvin scale is equal to 50°C?

Describe an activity to show that the rate of evaporation increases with surface area.

Answer:

1. 50+273= 323 K
2. Activity Take a small amount of water in three containers which have different surface areas. Keep them in sunlight for 2 h. Measure the volume of water left in all three containers. O
   1. Observation The amount of water left will be the least in a container having the largest surface area among them. Conclusion The Greater the surface area, more will be the rate of evaporation.

Class 9 Science Chapter 1 Matter In Our Surroundings Very Short Answer Type Questions

Question 1. A substance has no mass. Can we consider it a matter?
Answer: No, because matter has a definite mass.

Question 2. A given substance X has definite volume, but no definite shape and can diffuse easily. What is the physical state of a substance X?
Answer:

The physical state of a substance X is liquid because liquids do not have definite shapes, but have a definite volume and can diffuse easily.

Question 3. Rubber band changes its shape. Is it solid?
Answer:

Rubber band changes their shape under force and regain their shape after removing the force. Thus, it is a solid.

Question 4. Why do liquids take up the shape of the container in which they are kept?
Answer:

Forces of attraction are not very strong in liquids which is necessary to maintain their shape. Therefore, they acquire the shape of the container in which they are kept.

Question 5. Why do liquids have a mostly lower density than solids?
Answer:

Liquids have fewer forces of attraction (between their particles), hence less mass and more volume as compared to solids, thus they have less density than solids.

Question 6. What would be the effect of

1. Temperature and
2. The density of liquids on the rate of diffusion of liquids?

Answer:

1. On increasing temperature, the rate of diffusion of liquids increases.
2. The rate of diffusion is higher for a liquid having a lower density.

Question 7. Name the properties of gases that help aquatic plants and animals to survive in water.
Answer:

Diffusion, as oxygen diffuses in water, which aquatic plants and animals take in.

Question 8. Why a gas cylinder cannot be half-filled?
Answer:

Particles of gas move randomly at a very high speed and occupy all the space available to them at a very fast speed. Therefore, the gas cylinder cannot be half-filled according to the volume.

Question 9. Why do gases exert more pressure on the walls of the container than solids?
Answer:

In gases, the particles move randomly at high speed and they collide with each other and also with the walls of the container. Thus, they exert more pressure on the walls of the container than solids.

Question 10. Which characteristic of a gas is used in supplying oxygen cylinders to hospitals?
Answer:

Gases are highly compressible and can be liquefied. Due to these properties, gases are used in supplying oxygen cylinders to hospitals.

Question 11. What is the full form of CNG? Mention its one property which makes it so important.
Answer:

The full form of CNG is Compressed Natural Gas. It is a clean fuel which does not create pollution on combustion.

Question 12. Mention two ways to liquefy atmospheric gases.
Answer:

The two ways to liquefy atmospheric gases are:

1. Increasing pressure
2. Decreasing temperature

Question 13. At what temperature, do ice (solid) and water (liquid) co-exist together?
Answer:

At 0°C or 273 K (melting point of ice or freezing point of water), both ice (solid) and water (liquid) can co-exist.

Question 14. Why is boiling called a bulk phenomenon?
Answer:

Since, boiling starts from the bulk, i.e. inside the liquid, therefore it is a bulk phenomenon.

Question 15. Why does the temperature remain constant during sublimation?
Answer:

During the process of sublimation, heat given to the system is used to evaporate solid into vapour at constant temperature. B and C both are liquids at room temperature. However, B’s boiling point is quite close to the room temperature and it is therefore volatile.

Question 16. On suffering from fever which will lower your body temperature more, ice or ice cold water? Why?
Answer:

Ice will lower the body temperature more than ice-cold water because the latent heat of the fusion of ice is quite high (335 J/kg). Ice is, therefore, expected to absorb more heat energy from the body and will lower the body temperature more than ice-cold water.

Question 17. Water as ice has a cooling effect, whereas water as steam may cause severe burns. Explain these observations.
Answer:

When ice melts, it absorbs the energy equal to the latent heat of fusion from the surroundings, therefore, causing a cooling effect. But steam releases the extra heat (equal to the latent heat of vapourisation) which it has absorbed when water is converted into steam. So, steam produces severe burns.

Structure Of The Atom Multiple Choice Questions

Question 1. The cathode ray experiment was done for the first time by

1. Goldstein
2. J. J. Thomson
3. Dalton
4. Rutherford

Answer: 2. J. J. Thomson

J. J. Thomson, his first experiment was to build a cathode ray tube with a metal cylinder on the end.

Question 2. By whom was neutron discovered

1. J. Chadwick
2. Dalton
3. Bohr
4. Rutherford

Answer: 1. J. Chadwick

Neutron is another sub-atomic particle, discovered by J. Chadwick in 1932, It is represented by n. Neutrons are electrically neutral particles.

Question 3. ₁H¹ is named (termed) as

1. Protium
2. Deuterium
3. Tritium
4. Proton

Answer: 1. Protium

The name of the 1H1 is the protium.

Question 4. Which class is near the nucleus of the atom?

1. K-class (shell)
2. L-class (shell)
3. M-class (shell)
4. N-class (shell)

Answer: 1. K-class (shell)

K-class (shell) is the nearest the nucleus of the atom.

Question 5. How many neutrons are present in the nucleus of a hydrogen atom?

1. 1
2. 2
3. 3
4. 0

Answer: 4. 0

There are no neutrons in the nucleus of the hydrogen atom, hence mass number of hydrogen atoms is zero.

Question 6. Which one of the following elements has a 2, 8, 13, 1 electronic configuration?

1. Chromium
2. Copper
3. Silver
4. Palladium

Answer: 1. Chromium

The real electronic configuration of chromium is 2, 8, 13, 1 but according to Bohr, it’s 2, 8, 12, 2.

Question 7. The number of valence electrons present in Mg²⁺ ions is

1. 16
2. 8
3. 17
4. 18

Answer: 2. 8

Valency means several electrons are present in the outer shell.

For 12Mg(atom) =2, 8, 2, valence electrons = 7 But i2Mg+ 2 (ion) = 2, 8 valence electrons = 8

Question 8. Which of the following is the valency of an element, it has 2, 8, 2 electronic configurations.

1. 2
2. 4
3. 6
4. 0

Answer: 1. 2

The valency of an element is 2 because two electrons are present in the outer shell.

Question 9. Which one of the following is the electronic configuration of sodium?

1. 2, 8
2. 8, 2, 1
3. 2, 1, 8
4. 2, 8, 1

Answer: 4. 2, 8, 1

The atomic number of sodium is 11. So, electronic configuration ₁₁N = 2, 8,1.

Question 10. The isotope used in the treatment of goiter is

1. U-235
2. Iodine (I-131)
3. Co-60
4. Na-24

Answer: 2. Iodine (1-131)

An isotope of iodine (1-131) is used in the treatment of goiter.

Question 11. The nucleons are

1. Electrons are protons
2. Protons and neutrons
3. Electrons and neutrons
4. None of these

Answer: 2. Electrons are protons

Question 12. In Ruther Ford’s experiment a-particles were deflected because of

1. Electrostatic attraction between y particles and positively charged part of an atom
2. Electrostatic repulsion between particles and the positively charged part of an atom
3. Electrostatic repulsion between p-particles and positively charged part of an atom
4. None of these

Answer: 2. Electrostatic repulsion between particles and the positively charged part of an atom

Question 13. The properties of an atom depends on

1. Number of protons
2. Number of neutrons
3. Atomic mass
4. Electronic structure

Answer: 4. Electronic structure

Question 14. Which one of the following is free of charge?

1. Electron
2. Proton
3. Neutron
4. None of the above

Answer: 3. Neutron

Question 15. The atomic number of sodium is 11 and its mass number is 23. It has

1. 12 protons and 11 electrons
2. 11 neutrons and 12 protons
3. 11 electrons and 12 neutrons
4. 12 electrons and 11 neutrons

Answer: 3. 11 electrons and 12 neutrons

Question 16. The mass of the nucleus is

1. Number of protons in the nucleus
2. Number of neutrons in the nucleus
3. Number of nucleons in the nucleus
4. None of these

Answer: 3. Number of nucleons in the nucleus

Question 17. The isotope used to remove brain tumors and treatment of cancer is

1. U-235
2. iodine
3. Co-60
4. Na-24

Answer: 3. Co-60

Question 18. Among the following is an element of electronic configuration

1. 2, 8
2. 2, 6
3. 2, 8, 18
4. 2, 8, 1

Answer: 2. 2, 6

Question 19. 8016, 8017, and 8018 are known as

1. Equilibrium
2. Even factor
3. Isotope
4. None of these

Answer: 3. Isotope

Question 20. Which of the following has the largest (maximum) number of isotopes?

1. Polonium
2. Hydrogen
3. Uranium
4. Lead

Answer: 1. Polonium